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Research article

Molecular detection of Salmonella typhi from gallbladder tissue of cholecystitis patients and its relation to gallstone formation

Running title: Salmonella typhi and its relation to gallstone formation
  • Received: 25 June 2024 Revised: 13 September 2024 Accepted: 23 September 2024 Published: 30 September 2024
  • The gallbladder is often colonized by Salmonella during typhoid fever, and cholelithiasis contributes to many factors one of them is cholecystitis, which results from bacterial infections. This study aims to detect Salmonella typhi in the tissue of the gallbladder and find out its role in cholelithiasis. A total of 55 patients undergoing clinical and ultrasound examination were enrolled in this study, 43 with cholelithiasis and 12 without cholelithiasis, in which the gallbladder was taken as part of the surgical treatment for morbid obesity. DNA from tissue was extracted using QIAamp DNA Mini kit (Qiagen GmbH, Hilden, Germany) according to the manufacturer's recommendations; primer sets were used in this study that targeted fliC and SopB genes. A considerable number of cholelithiasis patients, 35 (81%), presented with multiple stone formation, while 8 (18.6%) presented with single stone formation. As many as 43 (78.1%) of cases presented with chronic cholecystitis, while only 12 (21.8%) of cases presented with acute cholecystitis by sonographic gallbladder; out of 55 samples, 10 (18.1%) were positive for the fliC gene in amplification of 367 bp. All 10 positive samples for Salmonella typhi showed gallbladder stone formation. However, out of 10 positive tissue samples for Salmonella typhi 10 (100%), of them were positive for the SopB gene. The findings in this study add to the body of knowledge around the occurrence rate of Salmonella typhi in gallbladder tissue samples obtained from patients with cholecystitis.

    Citation: Bashar A. Abdul Hassan, Jabbar S. Hassan, Ali Nayyef Umayra, Thanaa R. Abdulrahman. Molecular detection of Salmonella typhi from gallbladder tissue of cholecystitis patients and its relation to gallstone formation[J]. AIMS Medical Science, 2024, 11(4): 378-387. doi: 10.3934/medsci.2024026

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  • The gallbladder is often colonized by Salmonella during typhoid fever, and cholelithiasis contributes to many factors one of them is cholecystitis, which results from bacterial infections. This study aims to detect Salmonella typhi in the tissue of the gallbladder and find out its role in cholelithiasis. A total of 55 patients undergoing clinical and ultrasound examination were enrolled in this study, 43 with cholelithiasis and 12 without cholelithiasis, in which the gallbladder was taken as part of the surgical treatment for morbid obesity. DNA from tissue was extracted using QIAamp DNA Mini kit (Qiagen GmbH, Hilden, Germany) according to the manufacturer's recommendations; primer sets were used in this study that targeted fliC and SopB genes. A considerable number of cholelithiasis patients, 35 (81%), presented with multiple stone formation, while 8 (18.6%) presented with single stone formation. As many as 43 (78.1%) of cases presented with chronic cholecystitis, while only 12 (21.8%) of cases presented with acute cholecystitis by sonographic gallbladder; out of 55 samples, 10 (18.1%) were positive for the fliC gene in amplification of 367 bp. All 10 positive samples for Salmonella typhi showed gallbladder stone formation. However, out of 10 positive tissue samples for Salmonella typhi 10 (100%), of them were positive for the SopB gene. The findings in this study add to the body of knowledge around the occurrence rate of Salmonella typhi in gallbladder tissue samples obtained from patients with cholecystitis.



    The time-delay phenomenon is widely present in various engineering systems, and its existence will affect the stability and performance of the system. Therefore, the investigations into time-delay systems have been widely studied in the control field in recent years [1,2,3]. Many achievements have been obtained in the fields involving discrete-time systems [4,5,6], fuzzy systems [7,8,9], networked control systems [10,11], load frequency control systems [12,13,14,15], Markovian jump systems [16,17], Lur'e systems [18,19], and H filtering control [20,21,22].

    The Lyapunov–Razumikhin approach and the Lyapunov–Krasovskii (L–K) approach are the most widely used methods for addressing stability issues in time-delay systems. The Lyapunov–Razumikhin approach has achieved some advancements in deriving necessary and sufficient conditions for the stability of time-delay systems [23]. On the other hand, the L–K approach aims to construct an appropriate L–K functional to obtain less conservative stability criteria in the form of Linear Matrix Inequality (LMI) for time-delay systems, especially for time-varying delay systems [24,25,26,27]. It is worth mentioning that the L–K approach is frequently employed in combination with some integral inequality methods for bounding the integral terms in the derivatives delay-related L–K functionals [28,29,30,31,32]. Admittedly, recent research mainly focuses on reducing the conservativeness of the stability criteria by refining the structure of delay-related L–K functionals. An augmented L–K functional incorporating additional state information was presented in [21], enhancing the stability criteria's dependence on time delay. The augmented L–K functional constructed in [33] further considered the integrity of the information about delay intervals and gave less conservative results. An augmented L–K functional was also introduced in [27]. Unlike the L–K functionals in [21,33], it avoids the occurrence of high-order terms of variables in the derivative of the functional, thus easing the hardship of the solving process.

    In addition to various augmented L–K functionals mentioned above, the delay-segmentation-based piecewise L–K functionals have also been widely discussed in recent research. The delay segmenting method can augment the delay-interval-related information in the functional derivatives. Subsequently, it enables a more in-depth exploration of the functional decrease within each interval instead of merely considering its global decreasing property. Consequently, this functional effectively reduces the conservativeness of the system stability criterion. Delay-segmentation-based piecewise L–K functionals can be categorized into continuous and non-continuous piecewise L–K functionals. A time-delay-partitioning-based L–K functional was constructed in [34], which effectively relaxes the constraints on the stability criteria of the system. In [35], Han et al. established a non-continuous L–K functional and gave a delay-related stability criterion. Additionally, for time-varying delay systems, some quadratic terms with time-varying delay were introduced in [18] in constructing the L–K functional using an improved delay-segmentation method, proposing a novel delay-segmentation-based piecewise functional.

    Based on the domain of time-varying delay and its derivatives, the definition of allowable delay set (ADS) was given in [1]. Building upon this, an improved ADS was presented in [31], which optimized the stability criterion for linear systems with time-varying delay. However, the ADS given in [1] and [31] exhibit the coverage areas in the form of polygons, as indicated in [36]. However, this issue was further explored in [37], which gave ADS covering a complete ellipsoidal field by introducing specific periodically varying delays. Subsequently, an ADS partitioning approach was introduced in [38], which further refines the ADS through the application of the delay-segmenting method. By integrating this with the non-continuous L–K functional method, a stability criterion with reduced conservativeness was developed. However, the non-continuous piecewise function in [38] does not account for information regarding the delay segmentation and hinders potential improvements. Consequently, the existing criterion remains rather conservative.

    In recent years, the research for periodical time delay has aroused the interest of researchers, and it exists widely in some mechanical motions[39,40,41]. Combining delay-related L–K functional and a looped function, a stability criterion based on a periodically varying delay with monotone intervals was given in [37]. Based on this, a higher-order free-matrix-based integral inequality was introduced to optimize the stability criteria in [42], reducing the conservativeness of the stability criterion of the system. Furthermore, an exponential stability analysis of switching time-delay systems is presented in [43], which utilizes the symbolic transformation of delay derivatives as switching information. The stability of periodic time-delay systems is also studied in [38] by further partitioning the monotone intervals. In light of this, conducting in-depth research on periodic time-delay systems characterized by monotonic intervals on such a foundation holds significant and promising research prospects.

    This paper proposes an innovative delay-segmentation-based non-continuous piecewise L–K functional. In different segments, the construction of different L–K functionals is presented according to the intervals where the delay is located, and information regarding the delay-interval-segmentation is fully exploited in the functional of each segment. Therefore, the derived stability condition is less conservative. Finally, two numerical examples and a single-area load frequency control system are given to demonstrate that the proposed approach significantly reduces the conservativeness of the presented stability criterion.

    For brevity, the notations used in this paper are summarized in Table 1.

    Table 1.  Notations.
    Symbol Meaning
    N Natural numbers: {1,2,3,}
    Rn Euclidean space in n dimensions
    Rm×n All real matrices of dimension m×n
    Sn All symmetric matrices of dimension n×n
    Sn+ All positive definite symmetric matrices of dimension n×n
    1 Inverse of matrix
    T Transpose of matrix
    Sym{} The sum of matrix and its transpose
    * A symmetric component inside a symmetric matrix
    diag{} A matrix with a block diagonal structure

     | Show Table
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    Consider the following linear system with time-varying delay:

    {˙x(t)=Ax(t)+Arx(tr(t)),x(t)=φ(t),t[r2,0],  (2.1)

    where A,ArRn×n are system matrices, x(t)Rn is state vector, and r(t) represents the time-varying delay, which satisfies the following form derived from [37,38,41]:

    r(t)=r0+˜rf(Ωt), (2.2)

    where r0 is a constant, f:R[1,1] is a differentiable periodic function satisfying |f|1, whose each period contains a monotone increasing interval and a monotone decreasing interval, and parameters ˜r and Ω represent its amplitude and frequency, respectively. For r(t), r1 is the lower bound, and r2 is the upper bound. In addition, the derivative of r(t) is defined as ˙r(t), μ,μ are its lower and upper bounds. Therefore, r(t) and ˙r(t) are satisfied

    r1r(t)r2,  |˙r(t)|μ<1, (2.3)

    with r1=r0˜r, r2=r0+˜r and μ=˜rΩ.

    The ADS partitioning approach based on a periodical time-varying delay is discussed in [38]. There exists t2k1, t2k, t2k+1, kN, in the periodic function r(t), which are extreme values of r(t) satisfying r(t2k1)=r(t2k+1)=r1 and r(t2k)=r2. r(t) is monotone increasing in the intervals t[t2k1,t2k) and monotone decreasing in the intervals t[t2k,t2k+1).

    Then, we choose moving points ϱ1k[t2k1,t2k) and ϱ2k[t2k,t2k+1), ensuring that r(ϱ1k)=r(ϱ2k)=rϱ. The uncertain delay value rϱ=r1+ϱ(r2r1),ϱ[0,1] that satisfies r1rϱr2, and its value changes with the change of parameter ϱ. Depending on the value range of r(t), we obtain two delay-segmentation-based intervals: r(t)[r1,rϱ] and r(t)[rϱ,r2]. Thus, the following ADS αiβi,i=1,2 are obtained

    {α1[r1,rϱ]×[0,μ]        β1[rϱ,r2]×[0,μ],α2[r1,rϱ]×[μ,0]     β2[rϱ,r2]×[μ,0].  (2.4)

    Remark 2.1. The ADS described in formula (2.4) was initially presented in [38]. To underscore the efficacy of the enhanced functional put forward in this paper, we opt for the identical ADS to analyze the stability of the considered systems.

    To derive the main results, the following lemma needs to be introduced.

    Lemma 2.1. [31,32] If x(t)[ω1,ω2]Rn and κRm are continuous and differentiable, for matrices USn+, HRm×2n, the following inequalities hold:

    ω2ω1˙xT(s)U˙x(s)ds1ω2ω1κTϑTˆUϑκ, (2.5)
    ω2ω1˙xT(s)U˙x(s)ds(ω2ω1)κTHˆU1HTκ+2κTHϑκ, (2.6)

    where

    κ=col{x(ω2),x(ω1),1ω2ω1ω2ω1x(s)ds},ϑ=[˜eT1˜eT2  ˜eT1+˜eT22˜eT3]T,ˆU=diag{U,3U},˜eθ=[0n×(θ1)n  In  0n×(3i)n],θ=1,,3.

    This section discusses the stability criterion for system (2.1) based on ADS . The subsequent notations are introduced to represent the vectors and matrices for convenience.

    δ1(t)=[xT(t)   xT(tr1)   xT(tr(t))   xT(tr2)   xT(trϱ)]T,δ2(t)=[˙xT(tr1)  ˙xT(tr(t))  ˙xT(tr2)  ˙xT(trϱ)]T,δ3(t)=ttr1x(s)ds, δ4(t)=tr1tr(t)x(s)ds, δ5(t)=tr(t)trϱx(s)ds,δ6(t)=tr(t)tr2x(s)ds, δ7(t)=tr1trϱx(s)ds, δ8(t)=trϱtr2x(s)ds,ϖ0(t)=[δT1(t)   δT3(t)]T,  ϖ1(s)=[xT(s)   ˙xT(s)],ϖ2α(t)=[δT1(t)  δT3(t)   δT4(t)  δT5(t)  δT8(t)]T,ϖ2β(t)=[δT1(t)   δT3(t)   δT7(t)   δT5(t)   δT6(t)]T,ϖ(t)=[δT1(t)   δT2(t)   1r1δT3(t)  1r(t)r1δT4(t)  1rϱr(t)δT5(t)  1r2r(t)δT6(t)  1rϱr1δT7(t)  1r2rϱδT8(t)]T,eρ=[0n×(ρ1)n   In  0n×(15ρ)n],ρ=1,2,,15.

    As shown below, Theorem 3.1 provides a stability criterion for the system (2.1) based on ADS .

    Theorem 3.1. For scalars ϱ[0,1],r2r10,μ<1, suppose that there exist matrices PS9n+, F,Xαiς,XβiςS6n+, Qς,Sαiς,SβiςS2n+, Yς,Tαiς,TβiςSn+ satisfying the condition (3.1), Wαi1,Wαi2,Wβi1,Wβi2Sn, Gαiς,GβiςR15n×2n, Nαi1,Nαi2,Nβi1,Nβi2R15n×n,i=1,2,ς=1,,3. Then, system (2.1) is stable if LMIs (3.2)–(3.3) and (3.4)–(3.5) are satisfied at the vertices of αi and βi, respectively.

    Xα11Xβ11Xβ21Xα21, Xβ12Xβ22,Xβ23Xα23Xα13Xβ13, Xα22Xα12,Sα11Sβ11Sβ21Sα21, Sβ12Sβ22,Sβ23Sα23Sα13Sβ13, Sα22Sα12,Tα11Tβ11Tβ21Tα21, Tβ12Tβ22,Tβ23Tα23Tα13Tβ13, Tα22Tα12, (3.1)
    WYTαi1>0, WYTαi2>0, (3.2)
    [Υαi(r(t),˙r(t))r(t)r1Gαi1rϱr(t)Gαi2r2r1Gαi3ˆWYTαi100ˆWYTαi20ˆYT1]<0, (3.3)
     WYTβi1>0, WYTβi2>0, (3.4)
    [Υβi(r(t),˙r(t))rϱr1Gβi1r(t)rϱGβi2r2r(t)Gβi3ˆYT200ˆWYTβi10ˆWYTβi2]<0, (3.5)

    where

    Υαi(r(t),˙r(t))=Λ1+Λ2αi+Λ3αi+Λ4αi,Υβi(r(t),˙r(t))=Λ1+Λ2βi+Λ3βi+Λ4βi,Λ1=Sym{h1ΠT0FΠ1}+ΠT2Q1Π2+ΠT3(Q2Q1)Π3+ΠT4(Q3Q2)Π4ΠT5Q3Π5+r21ΠT20Y1Π20+(rϱr1)ΠT20Y2Π20+(r2rϱ)ΠT20Y3Π20,Λ2αi=Sym{ΠT0[(r(t)r1)Xαi1+(rαr(t))Xαi2+(r2rϱ)Xαi3]Π1+ΠT7PΠ8}+˙r(t)ΠT0(Xαi1Xαi2)Π0+ΠT3Sαi1Π3+(1˙r(t))ΠT6(Sαi2Sαi1)Π6+ΠT4(Sαi3Sαi2)Π4ΠT5Sαi3Π5+(r2rϱ)eT9(Tαi3Tαi2)e9+(r2r(t))(1˙r(t))eT7(Tαi2Tαi1)e7+(r2r1)eT6Tαi1e6,Λ3αi=(r(t)r1)((1˙r(t))eT7Wαi2e7eT9Wαi2e9)(rϱr(t))(eT6Wαi1e6(1˙r(t))eT7Wαi1e7),Λ2βi=Sym{ΠT0[(rϱr1)Xβi1+(r(t)rα)Xβi2+(r2r(t))Xβi3]Π1+ΠT9PΠ10}+˙r(t)ΠT0(Xβi2Xβi3)Π0+ΠT3Sβi1Π3+(1˙r(t))ΠT6(Sβi3Sβi2)Π6+ΠT4(Sβi2Sβi1)Π4ΠT5Sβi3Π5+(r2r(t))(1˙r(t))eT7(Tβi3Tβi2)e7+(r2rϱ)eT9(Tβi2Tβi1)e9+(r2r1)eT6Tβi1e6,Λ3βi=(r(t)rϱ)((1˙r(t))eT7Wβi2e7eT8Wβi2e8)(r2r(t))(eT9Wβi1e9(1˙r(t))eT7Wβi1e7),Λ4αi=Sym{Gαi1Π12+Gαi2Π13+Gαi3Π14+Nαi1Π18+Nαi2Π19}ΠT11ˆY1Π11,Λ4βi=Sym{Gβi1Π15+Gβi2Π16+Gβi3Π17+Nβi1Π18+Nβi2Π19}ΠT11ˆY1Π11,ˆYT1=diag{YT1,3YT1},  ˆYT2=diag{YT2,3YT2},ˆWYTαi1=diag{WYTαi1,3WYTαi1},  ˆWYTαi2=diag{WYTαi2,3WYTαi2},ˆWYTβi1=diag{WYTβi1,3WYTβi1},  ˆWYTβi2=diag{WYTβi2,3WYTβi2},

    with

    Π0=[eT1    eT2    eT3    eT4    eT5    r1eT10]T,    Π1=[ΠT20    eT6    (1˙r(t))eT7    eT8    eT9    r1eT10]T,Π2=[eT1    ΠT20]T,    Π3=[eT2    eT6]T,    Π4=[eT5    eT9]T,    Π5=[eT4    eT8]T,    Π6=[eT3    eT7]T,Π7=[eT1    eT2    eT3    eT4    eT5    r1eT10(r(t)r1)eT11    (rϱr(t))eT12    (r2rϱ)eT15]T,Π8=[ΠT20    eT6    (1˙r(t))eT7    eT8    eT9    eT1eT2    eT2(1˙r(t))eT3    (1˙r(t))eT3eT5    eT5eT4]T,Π9=[eT1    eT2    eT3    eT4    eT5    r1eT10    (rϱr1)(t)eT14    (r(t)rϱ)eT12    (r2r(t))eT13]T,Π10=[ΠT20    eT6    (1˙r(t))eT7    eT8    eT9    eT1eT2    eT2eT5    eT5(1˙r(t))eT3    (1˙r(t))eT3eT4]T,Π11=[eT1eT2    eT1+eT22eT10]T,    Π12=[eT2eT3    eT2+eT32eT11]T,Π13=[eT3eT5    eT3+eT52eT12]T,    Π14=[eT5eT4    eT5+eT42eT15]T,Π15=[eT2eT5    eT2+eT52eT14]T,    Π16=[eT5eT3    eT5+eT32eT12]T,Π17=[eT3eT4    eT3+eT42eT13]T,Π18=(r(t)r1)e11+(rϱr(t))e12(rϱr1)e14,Π19=(r(t)rϱ)e12+(r2r(t))e13(r2rϱ)e15,Π20=Ae1+Are3,YT1=Y3+Tαi3,  YT2=Y2+Tβi1,  ˆY1=diag{Y1,3Y1},WYTαi1=Y2+Tαi1˙r(t)Wαi1,  WYTαi2=Y2+Tαi2˙r(t)Wαi2,WYTβi1=Y3+Tβi2˙r(t)Wβi1,  WYTβi2=Y3+Tβi3˙r(t)Wβi2.

    Proof. Choosing the non-continuous piecewise L–K functional defined below:

    V(t)={V0(t)+V1α1(t)+V2α1+Vlα1(t), t[t2k1,ϱ1k),V0(t)+V1β1(t)+V2β1+Vlβ1(t), t[ϱ1k,t2k),V0(t)+V1β2(t)+V2β2+Vlβ2(t), t[t2k, ϱ2k),V0(t)+V1α2(t)+V2α2+Vlα2(t), t[ϱ2k, t2k+1),  (3.6)

    where

    V0(t)=r1ϖ0(t)TFϖ0(t)+ttr1ϖT1(s)Q1ϖ1(s)ds+tr1trϱϖT1(s)Q2ϖ1(s)ds+trϱtr2ϖT1(s)Q3ϖ1(s)ds+r10r1tt+u˙xT(s)Y1˙x(s)dsdu+r1rϱtt+u˙xT(s)Y2˙x(s)dsdu+rϱr2tt+u˙xT(s)Y3˙x(s)dsdu,V1αi(t)=ϖT0(t)[(r(t)r1)Xαi1+(rαr(t))Xαi2+(r2rϱ)Xαi3]ϖ0(t)+ϖT2α(t)Pϖ2α(t)+tr1tr(t)ϖT1(s)Sαi1ϖ1(s)ds+tr(t)trϱϖT1(s)Sαi2ϖ1(s)ds+trϱtr2ϖT1(s)Sαi3ϖ1(s)ds,V1βi(t)=ϖT0(t)[(rϱr1)Xβi1+(r(t)rα)Xβi2+(r2r(t))Xβi3]ϖ0(t)+ϖT2β(t)Pϖ2β(t)+tr1trϱϖT1(s)Sβi1ϖ1(s)ds+trϱtr(t)ϖT1(s)Sβi2ϖ1(s)ds+tr(t)tr2ϖT1(s)Sβi3ϖ1(s)ds,V2αi(t)=tr1tr(t)(r2+st)˙xT(s)Tαi1˙x(s)ds+tr(t)trϱ(r2+st)˙xT(s)Tαi2˙x(s)ds+trϱtr2(r2+st)˙xT(s)Tαi3˙x(s)ds,V2βi(t)=tr1trϱ(r2+st)˙xT(s)Tβi1˙x(s)ds+trϱtr(t)(r2+st)˙xT(s)Tβi2˙x(s)ds+tr(t)tr2(r2+st)˙xT(s)Tβi3˙x(s)ds,Vlαi(t)=(r(t)rϱ)tr1tr(t)˙xT(s)Wαi1˙x(s)ds+(r(t)r1)tr(t)trϱ˙xT(s)Wαi2˙x(s)ds,Vlβi(t)=(r(t)r2)trϱtr(t)˙xT(s)Wβi1˙x(s)ds+(r(t)rϱ)tr(t)tr2˙xT(s)Wβi2˙x(s)ds.

    Differentiating V(t) yields

    ˙V(t)={˙V0(t)+˙V1α1(t)++˙V2α1(t)+˙Vlα1(t), t[t2k1,ϱ1k),˙V0(t)+˙V1β1(t)+˙V2β1(t)+˙Vlβ1(t), t[ϱ1k,t2k),˙V0(t)+˙V1β2(t)+˙V2β2(t)+˙Vlβ2(t), t[t2k, ϱ2k),˙V0(t)+˙V1α2(t)+˙V2α2(t)+˙Vlα2(t), t[ϱ2k, t2k+1),  (3.7)

    where

    ˙V0(t)=2r1ϖT0(t)F˙ϖ0(t)+ϖT1(t)Q1ϖ1(t)+ϖT1(tr1)(Q2Q1)ϖ1(tr1)+ϖT1(trϱ)(Q3Q2)ϖ1(trϱ)ϖT1(tr2)Q3ϖ1(tr2)+r21˙xT(t)Y1˙x(t)+(rϱr1)˙xT(t)Y2˙x(t)+(r2rϱ)˙xT(t)Y3˙x(t)+ϝc1+ϝc2+ϝc3,˙V1αi(t)=2ϖT0(t)[(r(t)r1)Xαi1+(rαr(t))Xαi2+(r2rϱ)Xαi3]˙ϖ0(t)+˙r(t)ϖT0(t)(Xαi1Xαi2)ϖ0(t)+2ϖT2α(t)P˙ϖ2α(t)+ϖT1(tr1)Sαi1ϖ1(tr1)+(1˙r(t))ϖT1(tr(t))(Sαi2Sαi1)ϖ1(tr(t))+ϖT1(trϱ)(Sαi3Sαi2)ϖ1(trϱ)ϖT1(tr2)Sαi3ϖ1(tr2),˙V1βi(t)=2ϖT0(t)[(rϱr1)Xβi1+(r(t)rα)Xβi2+(r2r(t))Xβi3]˙ϖ0(t)+˙r(t)ϖT0(t)(Xβi2Xβi3)ϖ0(t)+2ϖT2β(t)P˙ϖ2β(t)+ϖT1(tr1)Sβi1ϖ1(tr1)+(1˙r(t))ϖT1(tr(t))(Sβi3Sβi2)ϖ1(tr(t))+ϖT1(trϱ)(Sβi2Sβi1)ϖ1(trϱ)ϖT1(tr2)Sβi3ϖ1(tr2),˙V2αi(t)=(r2rϱ)˙xT(trϱ)(Tαi3Tαi2)˙x(trϱ)+(r2r(t))(1˙r(t))˙xT(tr(t))T(Tαi2Tαi1)˙x(tr(t))+(r2r1)˙xT(tr1)Tαi1˙x(tr1)+ϝTαi1+ϝTαi2+ϝTαi3,˙V2βi(t)=(r2r(t))(1˙r(t))˙xT(tr(t))(Tβi3Tβi2)˙x(tr(t))+(r2rϱ)˙xT(trϱ)(Tβi2Tβi1)˙x(trϱ)+(r2r1)˙xT(tr1)Tβi1˙x(tr1)+ϝTβi1+ϝTβi2+ϝTβi3,˙Vlαi(t)=(r(t)r1)[(1˙r(t))˙xT(tr(t))Wαi2˙x(tr(t))˙xT(trϱ)Wαi2˙x(trϱ)]+(r(t)rϱ)[˙xT(tr1)Wαi1˙x(tr1)(1˙r(t))˙xT(tr(t))Wαi1˙x(tr(t))]+ϝWαi1+ϝWαi2,˙Vlβi(t)=(r(t)rϱ)[(1˙r(t))˙xT(tr(t))Wβi2˙x(tr(t))˙xT(tr2)Wβi2˙x(tr2)]+(r(t)r2)[˙xT(trϱ)Wβi1˙x(trϱ)(1˙r(t))˙xT(tr(t))Wβi1˙x(tr(t))]+ϝWβi1+ϝWβi2,

    with

    ϝc1=r1ttr1˙xT(s)Y1˙x(s)ds,  ϝc2=tr1trϱ˙xT(s)Y2˙x(s)ds,  ϝc3=trϱtr2˙xT(s)Y3˙x(s)ds,ϝTαi1=tr1tr(t)˙xT(s)Tαi1˙x(s)ds,    ϝTαi2=tr(t)trϱ˙xT(s)Tαi2˙x(s)ds,ϝTαi3=trϱtr2˙xT(s)Tαi3˙x(s)ds,    ϝTβi1=tr1trϱ˙xT(s)Tβi1˙x(s)ds,ϝTβi2=trϱtr(t)˙xT(s)Tβi2˙x(s)ds,    ϝTβi3=tr(t)tr2˙xT(s)Tβi3˙x(s)ds,ϝWαi1=˙r(t)tr1tr(t)˙xT(s)Wαi1˙x(s)ds,    ϝWαi2=˙r(t)tr(t)trϱ˙xT(s)Wαi2˙x(s)ds,ϝWβi1=˙r(t)trϱtr(t)˙xT(s)Wβi1˙x(s)ds,    ϝWβi2=˙r(t)tr(t)tr2˙xT(s)Wβi2˙x(s)ds.

    Applying (2.5) in Lemma 2.1 to estimate ϝc1, we obtain

    ϝc1ϖT(t)ET24Y1E24ϖ(t). (3.8)

    Then, merging the integrals that have the same integral intervals and applying (2.6) in Lemma 2.1 to estimate the various combinations of 3ι=2ϝcι+3ι=1ϝTαiι+2ι=1ϝWαiι.

    For ϝc2+ϝTαi1+ϝTαi2+ϝWαi1+ϝWαi2 and ϝc3+ϝTαi3, suppose that Y2+Tαi1+˙r(t)Wαi1>0 and Y2+Tαi2+˙r(t)Wαi2>0 at the vertices of αi, we obtain

    ϝc3+ϝTαi3=trϱtr2˙xT(s)(Y3+Tαi3)˙x(s)dsϖT(t)[(r2rϱ)Gαi3(Y3+Tαi3)1GTαi3+Sym{Gαi3E27}]ϖ(t), (3.9)
    ϝc2+ϝTαi1+ϝTαi2+ϝWαi1+ϝWαi2=tr1tr(t)˙xT(s)(Y2+Tαi1+˙r(t)Wαi1)˙x(s)dstr(t)trϱ˙xT(s)(Y2+Tαi2+˙r(t)Wαi2)˙x(s)dsϖT(t)[(r(t)r1)Gαi1(Y2+Tαi1+˙r(t)Wαi1)1GTαi1+(rϱr(t))Gαi2(Y2+Tαi2+˙r(t)Wαi2)1GTαi2+Sym{Gαi1E25+Gαi2E26}]ϖ(t). (3.10)

    Similarly, suppose that Y3+Tβi2+˙r(t)Wβi1>0 and Y3+Tβi3+˙r(t)Wβi2>0 at the vertices of βi. Then, ϝc3+ϝTβi2+ϝTβi3+ϝWβi1+ϝWβi2 and ϝc2+ϝTβi1 satisfy the following inequalities:

    ϝc2+ϝTβi1=tr1trϱ˙xT(s)(Y2+Tβi1)˙x(s)dsϖT(t)[(rϱr1)Gβi1(Y2+Tβi1)1GTβi1+Sym{Gβi1E28}]ϖ(t), (3.11)
    ϝc3+ϝTβi2+ϝTβi3+ϝWβi1+ϝWβi2=trϱtr(t)˙xT(s)(Y3+Tβi2+˙r(t)Wβi1)˙x(s)dstr(t)tr2˙xT(s)(Y3+Tβi3+˙r(t)Wβi2)˙x(s)dsϖT(t)[(r(t)rϱ)Gβi2(Y3+Tβi2+˙r(t)Wβi1)1GTβi2+(r2r(t))Gβi3(Y3+Tβi3+˙r(t)Wβi2)1GTβi3+Sym{Gβi2E29+Gβi3E30}]ϖ(t). (3.12)

    According to the relationship between the internal elements of the vector, the subsequent equations are valid.

    0=2ϖ(t)TNαi1[δ4(t)+δ5(t)δ7(t)],0=2ϖ(t)TNαi2[δ6(t)δ5(t)δ8(t)],0=2ϖ(t)TNβi1[δ4(t)+δ5(t)δ7(t)],0=2ϖ(t)TNβi2[δ6(t)δ5(t)δ8(t)]. (3.13)

    Combining (3.7)–(3.10) and (3.13), for t[t2k1,ϱ1k)[ϱ2k,t2k+1), we derive

    ˙V(t)ϖT(t)[Υαi(r(t),˙r(t))+(r(t)r1)Gαi1(Y2+Tαi1+˙r(t)Wαi1)1αi1T+(rϱr(t))Gαi2(Y2+Tαi2+˙r(t)Wαi2)1GTαi2+(r2rϱ)Gαi3(Y3+Tαi3)1GTαi3]ϖ(t)=ϖT(t)Ξαi(r(t),˙r(t))ϖ(t), (3.14)

    where Υαi(r(t),˙r(t)) is defined in Theorem 3.1. From the Schur complement lemma, Ξαi(r(t),˙r(t))<0 is equivalent to LMIs (3.3) at the vertices of αi. Therefore, there exist scalars γαi satisfying ˙V(t)<γαi|x(t)|2 for t[t2k1,ϱ1k)[ϱ2k,t2k+1) if (3.3) is satisfied.

    Similarly, for t[ϱ1k,ϱ2k), combining (3.7), (3.8), (3.11)–(3.13), we derive

    ˙V(t)ϖT(t)[Υβi(r(t),˙r(t))+(rϱr1)Gβi1(Y2+Tβi1)1GTbβi1+(r(t)rϱ)Gβi2(Y3+Tβi2+˙r(t)Wβi1)1GTβi2+(r2r(t))Gβi3(Y3+Tβi3+˙r(t)Wβi2)1GTβi3]ϖ(t)=ϖT(t)Ξβi(r(t),˙r(t))ϖ(t), (3.15)

    where ϖT(t)Ξβi(r(t),˙r(t))ϖ(t)<0 corresponds to LMIs (3.5) at the vertices of βi and there exist scalars γβi satisfying ˙V(t)<γβi|x(t)|2 for t[ϱ1k,ϱ2k) if (3.5) are satisfied.

    To ensure the overall decrement of the functional, some boundary conditions shown below need to be satisfied at each segmented point:

    {limtϱ 1k[V1α1(t)+V2α1(t)][V1β1(ϱ1k)+V2β1(ϱ1k)],limtt 2k[V1β1(t)+V2β1(t)][V1β2(t2k)+V2β2(t2k)],limtϱ 2k[V1β2(t)+V2β2(t)][V1α2(ϱ2k)+V2α2(ϱ2k)],limtt 2k+1[V1α2(t)+V2α2(t)][V1α1(t2k1)+V2α1(t2k1)]. (3.16)

    From which we obtain the restriction in (3.1). Thus, the proof is completed.

    Remark 3.1. An ADS based on the delay segmenting method is proposed in [38]. However, the L–K functional established therein does not match the ADS appropriately, i.e., the delay-segmentation-related information is not directly reflected in the established L–K functional. Undoubtedly, this makes the stability criterion of the system have a relatively high conservativeness in an intuitive way. Therefore, an improved L–K functional is established in this paper, which enables the direct manifestation of the segmented delay intervals. Compared with the functional established in [38], this functional increases the weight of delay derivatives in the criterion, thereby enhancing the correlation between the system stability conditions and the delay-related information. Moreover, as shown in V1αi and V1βi, the number of relevant Lyapunov matrices it encompasses has increased by 50%, while the number of constraint conditions between Lyapunov matrices has only increased by 33.33%. As shown in constraint conditions (3.1) of Theorem 3.1, Xα12Xβ12, Xβ22Xα22, Sα12Sβ12, and Sβ22Sα22 are not required. Consequently, the function presented in this paper will further reduce the conservativeness of the results.

    Remark 3.2. In [38], only two delay-product terms were incorporated in the established functional, aside from loop-like functions. This suggests that the delay-dependent stability conditions across different intervals will rely on a fixed set of Lyapunov matrices. Consequently, the stability criterion derived from [38] tends to produce overly conservative results. In contrast, the functional introduced in this paper incorporates V2αi and V2βi based on the specific intervals associated with the delay. This approach allows for the Lyapunov matrices used in the directly delay-dependent stability conditions across different intervals to differ from one another. As a result, the proposed function grants the stability criteria of the system greater flexibility and significantly diminishes its conservativeness.

    Remark 3.3. A loop function based on periodic time delay was originally proposed in [37]. This function exhibits the property of being identically zero at the vertices of the domain of the delay function. Consequently, when integrated with the L–K functional, it inherently satisfies the requirements of Lyapunov functions. Moreover, within distinct intervals of the functional, the Lyapunov matrices associated with this function are not required to be identical. However, the ADS partitioning approach introduced in [38] leads to the derivation of stability conditions of more meticulous segmentation from the proposed functional. As a result, constraints are imposed on the Lyapunov matrices within the loop functions. This imposition undermines the definition of the loop functions and increases the conservativeness of the system stability criteria. In contrast, in the functional presented herein, a set of loop functions corresponding to the delay-belonging intervals within each segment of the functional is established. These loop functions are valid on every individual segment. For t[t2k1,ϱ1k), given that V(t2k1)0, V(ϱ1k)0, and ˙V(t)<γα1|x(t)|2, then V(t2k1)V(t)V(ϱ1k) holds true. For other intervals, similar conditions also hold. Therefore, there are no restrictive relationships among their respective Lyapunov matrices. Evidently, this configuration further diminishes the conservativeness of the derived stability conditions.

    Remark 3.4. As evident from LMIs (3.3) and (3.5), the stability criterion derived from Theorem 3.1 encompasses a substantial number of variables (NoVs). To minimize computational complexity to the greatest extent possible, conserve computational resources, and investigate the correlation between the increment in computational complexity and the results enhancement, we reduce the number of free matrices in Eq (3.13). Subsequently, we substitute Eq (3.13) with the following equations and apply them to Theorem 3.1,

    0=2ϖ(t)TNα[δ4(t)+δ5(t)δ7(t)],0=2ϖ(t)TNβ[δ6(t)δ5(t)δ8(t)]. (3.17)

    The results obtained from these modifications will be compared and analyzed in the section dedicated to numerical examples.

    Then, the following corollary is derived based on a continuous L–K functional simplified by (3.6).

    Corollary 3.1. For scalars ϱ[0,1],r2r10,μ<1, if there exist matrices PS9n+, F,ˉXςS6n+, Qς,ˉSςS2n+, Yς,ˉTςSn+, Wαi1,Wαi2,Wβi1,Wβi2Sn, Gαiς,GβiςR15n×2n, Nαi1,Nαi2,Nβi1,Nβi2R15n×n,i=1,2,ς=1,,3, such that LMIs (3.18)–(3.19) and (3.20)–(3.21) are satisfied at the vertices of αi and βi, respectively, then system (2.1) is stable,

    WYˉTαi1>0, WYˉTαi2>0, (3.18)
    [ˉΥαi(r(t),˙r(t))r(t)r1Gαi1rϱr(t)Gαi2r2r1Gαi3ˆWYˉTαi100ˆWYˉTαi20ˆYˉT1]<0, (3.19)
    WYˉTβi1>0, WYˉTβi2>0, (3.20)
    [ˉΥβi(r(t),˙r(t))rϱr1Gβi1r(t)rϱGβi2r2r(t)Gβi3ˆYˉT200ˆWYˉTβi10ˆWYˉTβi2]<0, (3.21)

    where

    ˉΥαi(r(t),˙r(t))=Λ1+ˉΛ2α+Λ3αi+Λ4αi,ˉΥβi(r(t),˙r(t))=Λ1+ˉΛ2β+Λ3βi+Λ4βi,Λ1=Sym{h1ΠT0FΠ1}+ΠT2Q1Π2+ΠT3(Q2Q1)Π3+ΠT4(Q3Q2)Π4ΠT5Q3Π5+r21ΠT20Y1Π20+(rϱr1)ΠT20Y2Π20+(r2rϱ)ΠT20Y3Π20,ˉΛ2α=Sym{ΠT0[(r(t)r1)ˉX1+(rαr(t))ˉX2+(r2rϱ)ˉX3]Π1+ΠT7PΠ8}+˙r(t)ΠT0(ˉX1ˉX2)Π0+ΠT3ˉS1Π3+(1˙r(t))ΠT6(ˉS2ˉS1)Π6+ΠT4(ˉS3ˉS2)Π4ΠT5ˉS3Π5+(r2rϱ)eT9(ˉT3ˉT2)e9+(r2r(t))(1˙r(t))eT7(ˉT2ˉT1)e7+(r2r1)eT6ˉT1e6,ˉΛ2β=Sym{ΠT0[(rϱr1)ˉX1+(r(t)rα)ˉX2+(r2r(t))ˉX3]Π1+ΠT9PΠ10}+˙r(t)ΠT0(ˉX2ˉX3)Π0+ΠT3ˉS1Π3+(1˙r(t))ΠT6(ˉS3ˉS2)Π6+ΠT4(ˉS2ˉS1)Π4ΠT5ˉS3Π5+(r2r(t))(1˙r(t))eT7(ˉT3ˉT2)e7+(r2rϱ)eT9(ˉT2ˉT1)e9+(r2r1)eT6ˉT1e6,Λ3αi=(r(t)r1)((1˙r(t))eT7Wαi2e7eT9Wαi2e9)(rϱr(t))(eT6Wαi1e6(1˙r(t))eT7Wαi1e7),Λ3βi=(r(t)rϱ)((1˙r(t))eT7Wβi2e7eT8Wβi2e8)(r2r(t))(eT9Wβi1e9(1˙r(t))eT7Wβi1e7),Λ4αi=Sym{Gαi1Π12+Gαi2Π13+Gαi3Π14+Nαi1Π18+Nαi2Π19}ΠT11ˆY1Π11,Λ4βi=Sym{Gβi1Π15+Gβi2Π16+Gβi3Π17+Nβi1Π18+Nβi2Π19}ΠT11ˆY1Π11,ˆYˉT1=diag{YˉT1,3YˉT1},  ˆYˉT2=diag{YˉT2,3YˉT2},ˆWYˉTαi1=diag{WYˉTαi1,3WYˉTαi1},  ˆWYˉTαi2=diag{WYˉTαi2,3WYˉTαi2},ˆWYˉTβi1=diag{WYˉTβi1,3WYˉTβi1},  ˆWYˉTβi2=diag{WYˉTβi2,3WYˉTβi2},

    with

    YˉT1=Y3+ˉT3,  YˉT2=Y2+ˉT1,WYˉTαi1=Y2+ˉT1˙r(t)Wαi1,  WYˉTαi2=Y2+ˉT2˙r(t)Wαi2,WYˉTβi1=Y3+ˉT2˙r(t)Wβi1,  WYˉTβi2=Y3+ˉT3˙r(t)Wβi2,

    and Πλ(λ=0,,20), ˆY1 are defined in Theorem 3.1.

    Proof. Setting ˉXς=Xαiς=Xβiς, ˉSς=Sαiς=Sβiς and ˉTς=Tαiς=Tβiς in L–K functional (3.6), we obtain a continuous piecewise L–K functional satisfying the following equations:

    {limtϱ 1kVlα1(t)=Vlβ1(ϱ1k),limtt 2kVlβ1(t)=Vlβ2(t2k),limtϱ 2kVlβ2(t)=Vlα2(ϱ2k),limtt 2k+1Vlα2(t)=Vlα1(t2k1). (3.22)

    The proof employs a procedure analogous to that in Theorem 3.1, and the detail will not be repeated here.

    Remark 3.5. To show the advantage of the non-continuous functional (3.6), Corollary 3.1 provides a stability condition that is derived by using the continuous functional (3.22). It will be shown in the section of numerical examples that the non-continuous functional (3.6) plays an important role in the reduction of conservativeness.

    In this section, we will verify the validity and superiority of the new results obtained in the previous section through two numerical examples and a single-area load frequency control system provided.

    Example 4.1 Consider the system (2.1) with

    A=[2.00.00.00.9],   Ar=[1.00.01.01.0].

    To highlight the effectiveness of our novel function for improving the results, we choose [37,38] to compare with our results in this example. Both studies adopt a similar bounding method to ours, utilizing the first-order Bessel-Legendre inequality along with the free-matrix-based inequality, and we adopt the optimal solutions for the results of [38] under the β changes. From Table 2, for various μ with r1=0, we obtain larger allowable upper bounds (AUBs) of r2 under the appropriate ϱ (the value of ϱ is accurate to the percentile) by applying Theorem 3.1 and corresponding NoVs.

    Table 2.  The AUBs of r2 for various μ with r1=0.
    Methods μ=0.1 μ=0.2 μ=0.5 μ=0.8 NoVs
    [37] Theorem 1 5.10 4.57 3.78 3.38 131.5n2+9.5n
    [38] Corollary 7 4.82 4.13 3.13 2.70 76.5n2+11.5n
    [38] Theorem 3 5.44 5.00 4.18 3.66 259.5n2+32.5n
    Corollary 3.1 4.95(ϱ=0.21) 4.30(ϱ=0.05) 3.37(ϱ=0.95) 2.98(ϱ=0.89) 317.5n2+25.5n
    Theorem 3.1 5.61(ϱ=0.55) 5.29(ϱ=0.56) 4.40(ϱ=0.40) 3.86(ϱ=0.18) 556n2+70n

     | Show Table
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    Further, when μ=0.2, the result of Theorem 3.1 exhibits a 5.8% improvement compared to the result of Theorem 3 in [38]. Additionally, when μ=0.5, the result of Theorem 3.1 shows a 16.4% enhancement over the result reported in [37]. On the other hand, we note that the result of Theorem 3.1 yields a larger NoVs than those presented in the listed literature. Indeed, it is inescapable that the computational complexity escalates as the conservativeness of the results diminishes. To further emphasize the reduction in the conservativeness of the results yielded by our proposed method, we will conduct verifications using other examples in the subsequent contents.

    In addition, compared with the results of Corollary 7 in [38] obtained by a continuous functional, Corollary 1 in this paper also gives higher AUBs in Table 2. When μ=0.8, Corollary 3.1 presents a 10.3% improvement compared to the result of Corollary 7 in [38].

    Next, we assign the initial value of the system state as x0(t)=[1  1]T and choose r(t)=5.29/2+(5.29/2)cos(0.4t/5.29), which satisfies r(t)[0,5.29] and ˙r(t)[0.2,0.2]. As shown in Figure 1, the state response depiction of the system illustrated in Example 4.1 demonstrates that each state component converges towards zero. This outcome further reinforces the validity of the derived conclusions.

    Figure 1.  State responses of the single-area load frequency control system.

    Example 4.2. Consider the system (2.1) with

    A=[0.01.01.02.0],   Ar=[0.00.01.01.0].

    In this example, we further present the simulation results of Theorem 3.1 and Remark 3.4 for various μ and r1 in Table 3 and compare them with [38,43]. Generally speaking, it is readily apparent that the results derived from Theorem 3.1 are notably superior to those presented in the existing literature, showcasing a substantial improvement. For r1=0, the result of Theorem 3.1 is 92% better than that of Theorem 3 in [38] for the case of slow delay (μ=0.1), although the NoVs is nearly doubled. In the case of rapid delay (μ=0.8), the result of Theorem 3.1 also outperforms that of Theorem 3 in [38] by 27%. When r1=1, it is observable that although the improvement effect starts to diminish as the lower bound of the delay increases, the improvement can still reach 20% when μ=0.5. Meanwhile, compared with Theorem 4 in [43], Theorem 3.1 has comparable NoVs, yet its results are evidently less conservative.

    Table 3.  The AUBs of r2 for various μ and r1.
    Methods μ=0.1 μ=0.2 μ=0.5 μ=0.8 NoVs
    r1=0 [43] Theorem 4 9.94 4.46 3.33 521n2+27n
    [38] Theorem 3 13.67(ϱ=0.49) 9.18(ϱ=0.49) 5.02(ϱ=0.53) 3.18 (ϱ=0.51) 259.5n2+32.5n
    Theorem 3.1 26.49(ϱ=0.01) 13.16(ϱ=0.04) 5.93(ϱ=0.27) 4.05(ϱ=0.21) 556n2+70n
    Remark 3.4 26.33(ϱ=0.01) 13.14(ϱ=0.03) 5.93(ϱ=0.25) 4.05(ϱ=0.21) 466n2+70n
    r1=1 [38] Theorem 3 13.48(ϱ=0.43) 8.74(ϱ=0.45) 4.18(ϱ=0.57) 2.37(ϱ=0.55) 259.5n2+32.5n
    Theorem 3.1 15.55(ϱ=0.05) 9.05(ϱ=0.13) 5.03(ϱ=0.87) 2.81(ϱ=0.85) 556n2+70n
    Remark 3.4 15.55(ϱ=0.05) 9.04(ϱ=0.13) 5.03(ϱ=0.88) 2.81(ϱ=0.84) 466n2+70n

     | Show Table
    DownLoad: CSV

    Furthermore, the results presented in Remark 3.4 are tabulated in Table 3. Evidently, Remark 3.4 involves fewer NoVs compared to Theorem 3.1. Consequently, it exhibits a relatively lower computational complexity. Nevertheless, the results derived from Remark 3.4 are comparable to those obtained from Theorem 3.1.

    Subsequently, we set the initial state as x0(t)=[1  0]T and choose the delay function as r(t)=13.16/2+(13.16/2)cos(0.4t/13.16). This function satisfies the conditions r(t)[0,13.16] and ˙r(t)[0.2,0.2]. The state response is depicted in Figure 2. It is shown in the figure that the system is stable.

    Figure 2.  State responses of the single-area load frequency control system.

    Example 4.3. Consider the single-area load frequency control system, which can be expressed as the system (2.1) with

    A=[DM1M0001Tt1Tt01TgR01Tg0ν000],    Ar=[00000000νKpTg00KiTg0000],

    where D stands for the generator's damping coefficient, M is its moment of inertia, Tt and Tg represent the time constants corresponding to the turbine and governor, respectively, R refers to the speed drop, and v represents the frequency bias factor. Kp and Ki denote the proportional and integral gains of the PI controller, respectively. Let D=1.0,M=10,Tt=0.3,Tg=0.1,R=0.05,v=21,Kp=0.8,Ki={0.1,0.2,0.3} and r1=0,μ=0.1. The AUBs of r2 under the appropriate ϱ are shown in Table 4, demonstrating results that are evidently less conservative than those of Theorem 1 in [37] as well as Corollary 7 and Theorem 3 in [38]. Especially when integral gains Ki=0.1, the result shows a 30% improvement in the AUBs compared to Theorem 3 in [38]. To further confirm the validity of our results, setting Ki=0.1 and the initial state x0(t)=[1  0  0.5  1]T, and choosing r(t)=12.74/2+(12.74/2)cos(0.2t/12.74), which satisfies r(t)[0,12.74] and ˙r(t)[0.1,0.1], respectively, the state responses of the single-area load frequency control system are shown in Figure 3. Obviously, they tend to be stable.

    Table 4.  The AUBs of r2 for various Ki.
    Methods Ki=0.1 Ki=0.2 Ki=0.3
    [37] Theorem 1 9.55 5.31 3.74
    [38] Corollary 7 9.04 5.12 3.39
    [38] Theorem 3 9.77 5.61 3.84
    Corollary 3.1 11.01(ϱ=0.07) 5.95(ϱ=0.11) 3.85(ϱ=0.13)
    Theorem 3.1 12.74(ϱ=0.17) 6.38(ϱ=0.17) 4.05(ϱ=0.15)

     | Show Table
    DownLoad: CSV
    Figure 3.  State responses of the single-area load frequency control system.

    In this paper, an improved delay-segmentation-based non-continuous piecewise L–K functional is established. Then, a low-conservativeness stability criterion for linear systems with a periodical time-varying delay is presented based on the functional. Finally, we provide two simulation examples alongside an application to a single-area load frequency control system to demonstrate the effectiveness of the proposed approach.

    Wei Wang: Writing-review & editing, formal analysis, validation, conceptualization, funding acquisition; Chang-Xin Li: Writing-original draft, software, methodology, investigation; Ao-Qian Luo: Writing-review & editing; Hui-Qin Xiao: Writing-review & editing, supervision. All authors have read and approved the final version of the manuscript for publication.

    The authors declare that they have not used Artificial Intelligence tools in the creation of this article.

    This study is supported by the National Natural Science Foundation of China (No.62173136), the Natural Science Foundation of Hunan Province (Nos.2024JJ7130 and 2020JJ2013), and the Scientific Research and Innovation Foundation of Hunan University of Technology.

    The authors confirm that the data supporting the findings of this study are available within the article.

    The authors declare no conflict of interest.


    Acknowledgments



    The authors highly appreciate all patients enrolled in this study and the efforts of all staff in Al-Imammian Al-Kadhimein Medical City during sample collection.

    Ethics approval of research and informed consent



    The current study was approved by the Institutional Review Board (I.R.B.) in Al-Nahrain University College of Medicine” (in: Jan.-10th-2023/No.96), and we confirm that we have obtained the patients' informed consent and Ethical approval prior to study.

    Data availability



    Further data is available from the corresponding author on reasonable request.

    Conflict of interest



    The authors declare no conflict of interest.

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