
The predication of the helium diffusion concentration as a function of a source term in diffusion equation is an ill-posed problem. This is called inverse radiogenic source problem. Although some classical regularization methods have been considered for this problem, we propose two new fractional regularization methods for the purpose of reducing the over-smoothing of the classical regularized solution. The corresponding error estimates are proved under the a-priori and the a-posteriori regularization parameter choice rules. Some numerical examples are shown to display the necessarity of the methods.
Citation: Xuemin Xue, Xiangtuan Xiong, Yuanxiang Zhang. Two fractional regularization methods for identifying the radiogenic source of the Helium production-diffusion equation[J]. AIMS Mathematics, 2021, 6(10): 11425-11448. doi: 10.3934/math.2021662
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The predication of the helium diffusion concentration as a function of a source term in diffusion equation is an ill-posed problem. This is called inverse radiogenic source problem. Although some classical regularization methods have been considered for this problem, we propose two new fractional regularization methods for the purpose of reducing the over-smoothing of the classical regularized solution. The corresponding error estimates are proved under the a-priori and the a-posteriori regularization parameter choice rules. Some numerical examples are shown to display the necessarity of the methods.
The study of helium diffusion dynamics has attracted the attention of many scholars in recent years. Compared with other isotope systems, helium has a high yield. In addition, high-precision and high-sensitivity helium analysis is relatively easy, and the (U-Th)/He isotopes dating has reached a relatively low-temperature condition, which is very important for the study of thermochronology. In [1], the author gives the helium diffusion and low-temperature thermochronometry of apatite. The effects of long alpha-stopping distance on (U-Th)/He ages are studied in literature [2]. For the application of helium isotope as thermochronometer in terrestrial and extrater restrial materials, refer to the paper [3]. For more important applications of helium isotopes in physics, please refer to the literature[4,5,6].
This article will consider the prediction of helium concentration as a function of the spatial variable source term, which is closely related to helium isotope dating and is a low-temperature thermochronometry method. The helium production-diffusion model is as follows:
{∂u(r,t)∂t=a(t)[∂2u(r,t)∂r2+2r∂u(r,t)∂r]+f(r),t∈(0,T),0<r<R,u(r,0)=0,0<r<R,u(R,t)=0,t∈(0,T),limr→0u(r,t)bounded,t∈(0,T), | (1.1) |
where 0<a0≤a(t)≤a1(a0 and a1 are constants), r is the dimensional radial variable, R is the radius of the spherical diffusion domain, given the final observation of the helium concentration as follows
u(r,T)=g(r),0<r<R. | (1.2) |
The object is to reconstruct the source term f(r).
Inverse source problem is a typical ill-posed problem, which has been studied by many scholars. For the discussion of the existence, uniqueness and stability of solutions, please refer to reference [7,8,9,10,11]. In reference [12,13], Isakov has given some theoretical studies on the inverse source problem, and in [14], Isakov explains the general inverse problem of partial differential equations. In [15], Bao et al. has used the Tikhonov method and the truncation method with a-priori parameter choice rule to study the problem of the inverse radiogenic source identification problem. In [16], Zhang and Yan applied a-posteriori truncation method to the radiogenic source identification for the helium production-diffusion equation, which is closely connected to the helium isotopes dating as one method of the low-temperature thermochronometry. For more literature on numerical methods, see [17,18,19,20,21,22,23,24,25].
Regarding the research on the regularization theory of inverse radiogenic source problem, most of the results are discussed in the context of a-priori parameter selection, the numerical results of posterior parameters are more less. The a-priori method is based on the smoothness conditions of the solution, which is convenient for theoretical analysis, but it is difficult to verify. Therefore, in actual calculations, the regularization method of the posterior parameter selection rule is more widely used. In order to better identify the radiation source problem of helium production-diffusion Eq (1.1), we will give two fractional regularization methods, namely weighted fractional Tikhonov regularization method (WTRM) and fractional Landweber regularization method (FLRM). For related research on fractional regularization methods, please refer to the literatures [26,27,28,29,30,31,32]. Also, for the application of more fractional regularization methods please refer to [33,34,35,36].
The outline of the paper is as follows. In Section 2, we give some preliminary results; In order to overcome the difficulty, a novel a-priori bound is introduced. In Section 3, the ill-posedness of inverse radiogenic source promblem (1.1) is also given. In Section 4, we constructed the regularization solution by a WTRM. Moreover, we have performed a detailed convergence analysis and given the a-priori and a-posterior regularization parameters choice rule. In Section 5, we use the FLRM to give the regularization solution of the inverse source problem, and give the regularization parameter choice rule of the provided method and the corresponding error estimate. In Section 6, Numerical examples are given, and the numerical results show that the proposed method is accurate and effective. This article summarizes some general conclusions in Section 7.
The following lemmas will be used.
Lemma 2.1. Given 0<α≤1, for constants x>0, c_=R22a1π2 and ¯c=R2a0π2, a0 and a1 are the diffusivity [15], we hold the following inequality
¯cαx2c_α+1+μx2α+2≤c1μ−1α+1. | (2.1) |
where c1=c_αααα+1¯cα(α+1)>0 are independent of α,¯c,c_.
Proof. For 0<α≤1, we define the following function:
h1(x)=¯cαx2c_α+1+μx2α+2, |
where x>0, h1(x) has a unique point x0=(¯cα+1αμ)1α+1≥0 such that h′1(x0)=0.
Clearly we have
h1(x)≤h1(x0)=c_αααα+1¯cα(α+1)μ−1α+1:=c1μ−1α+1. |
Lemma 2.2. For constants x>0, 0<α≤1 and p>0, we have
μx2α+2−pc_α+1+μx2α+2≤{c2μp2α+2,0<p<2α+2,c3μ,p≥2α+2. | (2.2) |
where c2=¯c−p2(1−p2α+2)(p2α+2−p)p2α+2,c3=c_−(α+1).
Proof. Given the following function:
h2(x)=μx2α+2−pc_α+1+μx2α+2, |
where x>0.
If 0<p<2α+2, then limx→0h2(x)=limx→∞h2(x)=0. Thus there exists a x0=¯c12(2α+2−pμp)12α+2≥0 which is a global maximizer such that h′2(x0)=0, we have
h2(x)≤supx∈(0,∞)h2(x)≤h2(x0), |
Thus, we have
h2(x)≤h2(x0)=¯c−p2(1−p2α+2)(p2α+2−p)p2α+2μp2α+2:=c2μp2α+2. |
If p≥2α+2, for x≥1 then we have
h2(x)=1(¯cα+1+μx2α+2)xp−(2α+2)μ≤1¯cα+1+μx2α+2μ≤c_−(α+1)μ:=c3μ. |
Lemma 2.3. For constants x>0 and 0<α≤1, we have
¯cμx2α−pc_α+1+μx2α+2≤{c4μp+22α+2,0<p<2α,c5μ,p≥2α. | (2.3) |
where c4=¯c(p+2)2+p(2α−p)2α−p2α+2c_1+p2(2α+2),c5=¯cc_α+1>0.
Proof. Define the following function:
h3(x)=¯cμx2α−pc_α+1+μx2α+2. |
If 0<p<2α, then limx→0h3(x)=limx→∞h3(x)=0. Thus there exists a unique x0=c_12(2α−pμ(p+2))12α+2≥0 which is a global maximizer such that h′3(x0)=0, we have
h3(x)≤supx∈(0,∞)h3(x)≤h3(x0), |
Therefore, we have
h3(x)≤h3(x0)=¯c(p+2)2+p(2α−p)2α−p2α+2c_1+p2(2α+2)μp+22α+2:=c4μp+22α+2. |
If p≥2α, x≥1, we have
h3(x)=¯cμ(c_α+1+μx2α+2)xp−2α≤¯cc_α+1+μx2α+2μ≤¯cc_α+1μ:=c5μ. |
Lemma 2.4.[37,38] For 0<λ<1, υ>0,m∈N, let rm(λ):=(1−λ)m, there holds:
rm(λ)λυ≤θυ(m+1)−υ, |
where
θυ={1,0≤υ≤1,υυ,υ>1. |
Lemma 2.5. For kn>0 and 12<α<1, 0<βk2n<1,m≥1, we have
k−1n[1−(1−βk2n)m]α≤β12m12. | (2.4) |
Proof. We define two functions with τ2:=βk2n:
ψ(τ)=βτ−2[1−(1−τ2)m]2α, | (2.5) |
and
ϕ(τ)=τ−2[1−(1−τ2)m]2α. | (2.6) |
Obviously ψ(τ)=βϕ(τ). These two functions are continuous in τ∈(0,1).
For 12<α<1 and τ∈(0,1), using the Lemma 3.3 in [28], we have
ϕ(τ)≤m,ψ(τ)≤βm. |
Therefore,
k−1n[1−(1−βk2n)m]α≤β12m12. |
Lemma 2.6. For m≥1,kn>0,0<βk2n<1, we have
kp2n(1−βk2n)m≤c(β,p)m−p4:=c6m−p4, | (2.7) |
where the constant c6=(p4β)p4.
Proof. We introduce a new variable x:=k2n,x<1/β, and let
h4(x)=(1−βx)mxp4. |
It is easy to see that there exists a unique x0=zβ(z+m) with z=p4 such that h′4(x0)=0. We find that
h4(x)≤h4(x0)≤(1−zz+m)m(zβ(z+m))z<(zβ)z(1z+m)z<(zβ)z(1m)z:=c6m−p4. |
In this section, we derive an analytical solution for the inverse radiogenic source problem based on the eigenfunction expansion, and give an analysis on the ill-posedness of the inverse source problem (1.1).
Throughout this paper, the Hilbert space of square integrable functions on [0,R] is denoted as L2([0,R]). ⟨⋅,⋅⟩ and ‖⋅‖ are the inner product and norm on L2([0,R]) respectively, introduced as follows
⟨f,g⟩=∫R0f(r)g(r)dr,and‖u‖=(∫R0|f(r)|2dr)12. |
In order to solve the problem (1.1), we introduce a new function ω(r,t) under the substitution
ω(r,t)=ru(r,t). |
It follows from the inverse radiogenic source problem (1.1), that ω satisfies:
{∂ω(r,t)∂t=a(t)∂2ω(r,t)∂r2+rf(r),t∈(0,T),0<r<R,ω(r,0)=0,0<r<R,ω(R,t)=0t∈(0,T),limr→0ω(r,t)bounded,t∈(0,T). | (3.1) |
The corresponding final observation of the helium concentration becomes
ω(r,T)=rg(r),0<r<R. | (3.2) |
Applying the method of separation of variables, consider the solution of problem (3.1) of the form
ω(r,t)=X(r)Y(t), | (3.3) |
substitute it into the (3.1), we obtain the following Sturm-Liouville problem
X″(r)+λX(r)=0.0<r<R,X(R)=0,limr→0r−1X(r)bounded, | (3.4) |
where λ is an unknown constant.
Through calculation, we can get the eigenvalues of (3.4) as follows
λn=(nπR)2,n=1,2,⋯, | (3.5) |
and the corresponding eigenfunctions are
Xn(r)=sin(nπrR). | (3.6) |
From the orthogonality and completeness of the eigenfunction system {sin(nπrR)}∞n=1 in L2([0,R]), we get the solution ω(r,t) and the source term r1f(r) can be represented as
ω(r,t)=∞∑n=1Xn(r)Yn(t), | (3.7) |
rf(r)=∞∑n=1fnXn(r), | (3.8) |
where
fn=∫R0rf(r)sin(nπrR)dr∫R0sin2(nπrR)dr=2R∫R0rf(r)sin(nπrR)dr,n=1,2,…. | (3.9) |
Substituting (3.7) and (3.8) into (3.1), we have
Y′n(t)+λa(t)Yn(t)=fn,Yn(0)=0. |
Solving the above initial-value problem yields the solution
Yn(t)=fn∫t0e−λn∫tτa(s)dsdτ. |
Therefore, the solution of (3.1) can be written as the infinite series
ω(r,t)=∞∑n=1fnXn(r)∫t0e−λn∫tτa(s)dsdτ. | (3.10) |
Evaluating (3.10) at t = T on both sides and using the final helium concentration (3.2) give
ω(r,T)=ru(r,T)=rg(r)=∞∑n=1fnXn(r)∫T0e−λn∫Tτa(s)dsdτ. | (3.11) |
Define
φn(r)=√2RXn(r)=√2Rsin(nπrR). | (3.12) |
It is easy to verify that the eigenfunctions {φn(r)}∞n=1 form an orthonormal basis in L2([0,R]). Using the eigenfunctions as a basis, formula (3.8) can be rewritten as:
rg(r)=∞∑n=1∫T0e−λn∫Tτa(s)dsdτ⟨fr(r),φn(r)⟩φn(r). | (3.13) |
For convenience, we denote
fr(r)=rf(r)andgr(r)=rg(r). | (3.14) |
To get fr, define an operator K:fr→gr, then the inverse source problem can be represented by the following linear operator equation:
Kfr(r)=gr(r). | (3.15) |
Using (3.13), it holds
Kfr(r)=∞∑n=1kn⟨fr(r),φn(r)⟩φn(r)=gr(r), | (3.16) |
with
kn=∫T0e−λn∫Tτa(s)dsdτ,n=1,2,…. | (3.17) |
Therefore, the analytical solution of the inverse source problem is:
fr(r)=∞∑n=1k−1n⟨gr(r),φn(r)⟩φn(r). | (3.18) |
Because measurement errors exist in the data function gr, the solution has to be reconstructed from noisy data gδr which is assumed to satisfy
‖gδr−gr‖≤δ. | (3.19) |
Here δ>0 represents the noise level, and both gr(r) and gδr(r) are assumed to be functions in L2([0,R]).
To study the ill-posed nature of the inverse problem, it is sufficient to investigate the decay property of the eigenvalues. From [15], we can see that the upper and lower bounds of kn are as follows
c_n2≤kn≤¯cn2,asn→∞, | (3.20) |
where the constant c_=R22a1π2 and ¯c=R2a0π2.
From (3.20), we note that when n→∞, the eigenvalue kn→0[15,p7]. Fixed size data error can be amplified arbitrarily much by the factors k−1n. Therefore, the problem of identifying f(r) is ill-posed. In the following, we will use two fractional regularization methods to solve the inverse radiogenic source promblem.
To obtain the error estimates, it is necessary to assume certain regularity of the exact source function. Here we assume that there exists an a priori estimate for the source function fr(r), i.e.,
‖fr‖p≤E,forp>0, | (3.21) |
where E>0 is a constant. where the norm is defined in terms of the eigenfunctions
‖fr‖p=‖∞∑n=1np⟨fr,φn⟩φn‖. | (3.22) |
It is easy to check ‖fr‖0=‖fr‖.
In this section, we propose a WTRM to solve the ill-posed problem (1.1) and give convergence estimate under the a-priori regularization parameter choice rule.
Then we consider WTRM to solve the ill-posed problem, the regularization solution is
fδ,μr(r)=∞∑n=1kαnkα+1n+μ⟨gδr(r),φn(r)⟩φn(r), | (4.1) |
where μ>0 plays the role of regularization parameter, we define α as the fractional parameter. When α=1, it expresses the classic Tikhonov method. However, for 0<α<1 we can see it prevent the effect of oversmoothing and obtain a more accurate numerical results for the discontinuity of solution.
Theorem 4.1. Suppose the a priori condition (3.21) and the noise assumption (3.19) hold,
(1) If 0<p<2α+2 and choice μ=(δE)2α+2p+2, we have a convergence estimate
‖fδ,μr(r)−fr(r)‖≤(c1+c2)E2p+2δpp+2. | (4.2) |
(2) If p≥2α+2 and choice μ=(δE)α+1α+2, we have a convergence estimate
‖fδ,μr(r)−fr(r)‖≤(c1+c3)E1α+2δα+1α+2. | (4.3) |
Proof. By the triangle inequality, we know
‖fδ,μr(r)−fr(r)‖≤‖fδ,μr(r)−fμr(r)‖+‖fμr(r)−fr(r)‖=I1+I2. | (4.4) |
We first give the estimate of I1, with Lemma 2.1 and (3.19), we have
I1=‖fδ,μr(r)−fμr(r)‖=‖∞∑n=1kαnkα+1n+μ⟨gδr−gr,φn⟩rφn‖≤δsupn>0(¯cn2)α(c_n2)α+1+μ≤c1μ−1α+1δ. |
Now we estimate I2, by Lemma 2.2 and a priori bound condition (3.21), we can deduce that
I2=‖fμr(r)−fr(r)‖=‖∞∑n=1(1−kα+1nkα+1n+μ)k−1n⟨gr,φn⟩rφn‖≤‖∞∑n=1μn−pkα+1n+μnp⟨fr,φn⟩rφn‖≤Esupn>0μn2α+2−pc_α+1+μn2α+2≤{c2μp2α+2E,0<p<2α+2,c3μE,p≥2α+2. |
Combining the above two inequality and choose the regularization parameter μ, we obtain
‖fδ,μr(r)−fr(r)‖≤{(c1+c2)E2p+2δpp+2,0<p<2α+2,(c1+c3)E1α+2δα+1α+2,p≥2α+2. |
In this section, we give the regularization parameter choice rule of the posterior fractional regularization method. We can also obtain a convergence rate for the regularized solution (4.1) under this parameter choice rule. The most general a posteriori rule is the Morozov's discrepancy principle [39].
We use the discrepancy principle in the following form:
‖Kfδ,μr(r)−gδr(r)‖=τδ, | (4.5) |
where 0<α≤1,τ>1 is a constant, μ>0 is regularization parameter, K is defined by the operator Eq (3.15). According to the following lemma, we know there exists a unique solution for Eq (4.5) if 0<τδ<‖gδr‖.
Lemma 4.2. Let d(μ)=‖Kfδ,μr(r)−gδr(r)‖, then we have the following conclusions: (1) d(μ) is a continuous function; (2) limμ→0d(μ)=‖gδr(r)‖; (3) limμ→∞d(μ)=0; (4) d(μ) is a strictly decreasing function over (0,∞).
Proof. The proofs are straightforward results by virtue of
d(μ)=(∞∑n=1(μkα+1n+μ)2(gδr)2)12. |
Lemma 4.3. If μ is the solution of Eq (4.5), we also obtain the following inequality:
μ−1α+1≤{(c4τ−1)2p+2(Eδ)2p+2,0<p<2α,(c5τ−1)1α+1(Eδ)1α+1,p≥2α. | (4.6) |
Proof. From (4.5), and according to Lemma 4.2, we obtain
τδ=‖Kfδ,μr(r)−gδr(r)‖≤‖∞∑n=1μkα+1n+μ⟨gδr−gr,φn⟩rφn‖+‖∞∑n=1μkα+1n+μ⟨gδr,φn⟩rφn‖≤δ+‖∞∑n=1μknn−pkα+1n+μnpk−1n⟨gδr,φn⟩rφn‖≤δ+Esupn>0¯cμn2α−pc_α+1+μn2α+2. |
According to Lemma 2.3, we have
τδ≤δ+E{c4μp+22α+2,0<p<2α,c5μ,p≥2α. |
This yields
μ−1α+1≤{(c4τ−1)2p+2(Eδ)2p+2,0<p<2α,(c5τ−1)1α+1(Eδ)1α+1,p≥2α. |
Theorem 4.4. Suppose the a priori condition (3.21) and the noise assumption (3.19) hold, and the regularization parameter μ is chosen by discrepancy principle (4.5), then,
(1) If 0<p<2α, we have a convergence estimate
‖fδ,μr(r)−fr(r)‖≤(c1(c4τ−1)2p+2+(τ+1C_)pp+2)E2p+2δpp+2. | (4.7) |
(2) If p≥2α, we have a convergence estimate
‖fδ,μr(r)−fr(r)‖≤((c6τ−1)1α+1+(τ+1C_)αα+1λ2α−p2(α+1)1)E1α+1δαα+1. | (4.8) |
Proof. By the triangle inequality, we know
‖fδ,μr(r)−fr(r)‖≤‖fδ,μr(r)−fμr(r)‖+‖fμr(r)−fr(r)‖=I1+I2. | (4.9) |
(1) For 0<p<2α. We first give the estimate of I1, with Lemma 4.3 we have
I1=‖fδ,μr(r)−fμr(r)‖≤c1μ−1α+1δ≤c1(c3τ−1)2p+2E2p+2δpp+2. | (4.10) |
Now we estimate I2, by (3.19) and (3.21), we can deduce that
I2=‖fμr(r)−fr(r)‖=‖∞∑n=1μkα+1n+μk−1n⟨gr,φn⟩φn‖=‖∞∑n=1μknkα+1n+μ⟨fr,φn⟩φnkn‖≤‖∞∑n=1μknkα+1n+μ⟨fr,φn⟩φn‖pp+2⋅‖μknkα+1n+μ⟨fr,φn⟩φnkp+22n‖2p+2≤‖∞∑n=1μkα+1n+μ⟨gr,φn⟩φn‖pp+2⋅‖∞∑n=1μnpkp2n(kα+1n+μ)n−p⟨fr,φn⟩φn‖2p+2≤(‖∞∑n=1μkα+1n+μ⟨gr−gδr,φn⟩φn‖+‖∞∑n=1μkα+1n+μ⟨gδr,φn⟩φn‖)pp+2supn>0(n−p(c_n2)p2)2p+2E2p+2≤(τ+1c_)pp+2E2p+2δpp+2. | (4.11) |
Combining (4.9), (4.10) and (4.11), we obtain
‖fδ,μr(r)−fr(r)‖≤(c1(c4τ−1)2p+2+(τ+1c_)pp+2)E2p+2δpp+2. |
(2) For p≥2α. From (4.9), we first give the estimate of I1, with Lemma 4.2 we have
I1=‖fδ,μr(r)−fδr(r)‖≤c1δμ−1α+1≤(c5τ−1)1α+1E1α+1δαα+1. | (4.12) |
Then we estimate I2, by Lemma 2.3 and (3.21), we known
I2=‖fμr(r)−fr(r)‖=‖∞∑n=1μkα+1n+μk−1n⟨gr,φn⟩φn‖=‖∞∑n=1μknkα+1n+μ⟨fr,φn⟩φnkn‖≤‖∞∑n=1μknkα+1n+μ⟨fr,φn⟩φn‖αα+1⋅‖μknkα+1n+μ⟨fr,φn⟩φnkα+1n‖1α+1≤‖∞∑n=1μkα+1n+μ⟨gr,φn⟩φn‖αα+1⋅‖∞∑n=1μn−pkαn(kα+1n+μ)np⟨fr,φn⟩φn‖1α+1≤(‖∞∑n=1μkα+1n+μ⟨gr−gδr,φn⟩φn‖+‖∞∑n=1μkα+1n+μ⟨gδr,φn⟩φn‖)αα+1‖n−p(c_n2)αnp⟨fr,φn⟩φn‖1α+1≤(τ+1c_)αα+1E1α+1δαα+1. | (4.13) |
Combining (4.9), (4.12) and (4.13), we obtain
‖fδ,μr(r)−fr(r)‖≤((c5τ−1)1α+1+(τ+1c_)αα+1)E1α+1δαα+1. |
This completes the proof.
In this section, we propose a FLRM to solve the ill-posed problem (1.1) and give convergence estimate under the a-priori regularization parameter choice rule.
We denote regularization solution of FLRM with the noisy data as follows:
fδ,mr(r)=∞∑n=1[1−(1−βk2n)m]αk−1n⟨gδr(r),φn(r)⟩φn(r),12<α≤1, | (5.1) |
where m≥1 plays the role of regularization parameter, 0<β<2k2n, α is called the fractional parameter. When α=1, it is the classic Landweber iterative method.
Now we give the main result of this section.
Theorem 5.1. Suppose the a priori condition (3.21) and the noise assumption (3.19) hold, let m=⌊(Eδ)4p+2⌋, we have the convergence estimate
‖fδ,mr(r)−fr(r)‖≤(β12+c6c_−p2)E2p+2δpp+2, | (5.2) |
where ⌊s⌋ denotes the largest integer smaller than or equal to s, c5 are the positive constants depending on β,p,α, and c_.
Proof. By the triangle inequality, we know
‖fδ,mr(r)−fr(r)‖≤‖fδ,mr(r)−fmr(r)‖+‖fmr(r)−fr(r)‖=J1+J2. | (5.3) |
As in the estimate of I1, by Lemma 2.5 and (3.19), we have
J1=‖fδ,mr(r)−fmr(r)‖=‖∞∑n=1[1−(1−βk2n)m]αk−1n⟨gδr−gr,φn⟩φn‖≤δsupn>0k−1n[1−(1−βk2n)m]α≤β12m12δ. |
Now we estimate J2, by Lemma 2.6 and the a-priori bound condition (3.21), we can deduce that
J2=‖fmr(r)−fr(r)‖=‖∞∑n=1[1−[1−(1−βk2n)m]α]k−1n⟨gr,φn⟩φn‖≤‖∞∑n=1(1−βk2n)mn−pnpk−1n⟨gr,φn⟩φn‖≤Esupn>0(1−βk2n)mn−p≤c6c_−p2m−p4E. |
Combining the above two inequalities, we obtain
‖fδm(r)−f(r)‖≤β12m12δ+c6c_−p2m−p4E. |
Choose the regularization parameter m by
m=⌊(Eδ)4p+2⌋, |
then we have the following result
‖fδ,mr(r)−fr(r)‖≤(β12+c6c_−p2)E2p+2δpp+2. |
Due to the semi-convergence property of iterative methods for ill-posed problems, we need a reliable stopping rule for detecting the moment from convergence to divergence. In this section, we give the a-posteriori parameter choice rule for the FLRM. We can obtain a convergence rate for the regularized solution (4.1) under this parameter choice rule. The most general a-posteriori rule is the Morozov's discrepancy principle [39].
We use the discrepancy principle in the following form:
‖Kfδ,mr(r)−gδr(r)‖≤τδ, | (5.4) |
where τ>1 is a user-supplied constant independent on δ, m>0 is regularization parameter which makes (5.4) hold at the first time, K is defined by the operator Eq (3.15).
Lemma 5.2. Let d(m)=‖Kfδ,mr(r)−gδr(r)‖, then we have the following conclusions: (1) d(m) is a continuous function; (2) limm→0d(m)=‖gδr(r)‖; (3) limm→∞d(m)=0; (4) d(m) is a strictly decreasing function over (0,∞).
Proof. By (5.1) and (5.4), we have
d(m)=(∞∑n=1[1−[1−(1−βk2n)m]α]2(gδr)2)12. |
Obviously, limm→0d(m)=(∑∞n=1(gδr)2)12=‖gδr(r)‖. Therefore the conclusions (1)–(4) are obvious.
Remark 5.3. We assume that the noisy data ‖gδr‖ is large enough such that 0<τδ<‖gδr‖, thus according to Lemma 5.2, there exists a unique minimum solution for inequality (5.4).
Lemma 5.4. If m is the solution of Eq (5.4), we can obtain the following inequality:
(mβ)12≤(θp+24c_p2(τ−1))2p+2(Eδ)2p+2, | (5.5) |
where
θp+24={1,0≤p≤2,(p+24)p+24,p>2. |
Proof. From (5.4), and according to Lemma 2.4, we obtain
τδ≤‖Kfδ,mr(r)−gδr(r)‖=‖∞∑n=1[1−[1−(1−βk2n)m−1]α]⟨gδr,φn⟩φn‖≤‖∞∑n=1(1−βk2n)m−1⟨gδr−gr,φn⟩φn‖+‖∞∑n=1(1−βk2n)m−1⟨gδr,φn⟩φn‖, |
then
τδ≤δ+Esupn>0(1−βk2n)m−1knn−p≤δ+c_−p2Esupn>0(1−βk2n)m−1(βk2n)p+24β−p+24≤δ+c_−p2θp+24(mβ)−p+24E. |
This yields
(mβ)12≤(θp+24c_p2(τ−1))2p+2(Eδ)2p+2. | (5.6) |
Theorem 5.5. Suppose the a priori condition (3.21) and the noise assumption (3.19) hold, then we have the convergence estimate
‖fδ,mr(r)−fr(r)‖≤((θp+24c_p2(τ−1))2p+2+(τ+1c_)pp+2)E2p+2δpp+2. |
Proof. By the triangle inequality, we know
‖fδ,mr(r)−fr(r)‖≤‖fδ,mr(r)−fmr(r)‖+‖fmr(r)−fr(r)‖=J1+J2. | (5.7) |
On the estimate of J1, by Lemma 5.4, we have
J1=‖fδ,mr(r)−fmr(r)‖≤β12m12δ≤(θp+24c_p2(τ−1))2p+2E2p+2δpp+2. | (5.8) |
Now we estimate J2, by the a priori bound condition (3.21), we can deduce that
J2=‖fmr(r)−fr(r)‖=‖∞∑n=1[1−[1−(1−βE2α,1(−λnTα))m]α]fnφn‖≤‖∞∑n=1(1−βk2n)m⟨fr,φn⟩φn‖≤‖∞∑n=1(1−βk2n)m⟨fr,φn⟩φn‖pp+2‖∞∑n=1(1−βk2n)mn−pnp⟨fr,φn⟩φn‖2p+2, | (5.9) |
where we have used the Hölder inequality. Therefore, by the triangle inequality, we obtain
J2≤(‖∞∑n=1(1−βk2n)mk−1n⟨(gr−gδr),φn⟩φn‖+‖∞∑n=1(1−βk2n)mk−1ngδnφn‖)pp+2‖∞∑n=1n−pnp⟨(fr,φn⟩φn‖2p+2≤(δ+τδ)pp+2supn>0(knn2)−pp+2≤(τ+1c_)pp+2E2p+2δpp+2. | (5.10) |
Combining (5.7), (5.8) and (5.10), we can get the conclusion.
In this section, three simple numerical examples are presented to show the validity of the two fractional regularization methods. the simulated data aregenerated as:
gδ=g(1+δ⋅randn(size(g))). |
where g is the solution of the forward problem, and randn is the white noise, the magnitude δ indicates the noise level of the measured data. In the numerical experiment, we use the spatial discretization number M=401, and we fix a(t)=1,T=1,R=1. f(r) is the exact solution, fδ,μ(r) and fδ,m(r) are regularized solutions of the WTRM and FLRM, respectively. The relative error calculations of the WTRM and FLRM are as follows
RE(WTRM)=√M∑i=1r(i)(f(i)−fδ,μ(i))2/√M∑i=1r(i)(f(i))2, |
RE(FLRM)=√M∑i=1r(i)(f(i)−fδ,m(i))2/√M∑i=1r(i)(f(i))2, |
where ‖⋅‖ is the L2([0,R]) norm.
In order to obtain the artificial data gδ, we need to solve the following forward problem:
{∂u(r,t)∂t=a(t)[∂2u(r,t)∂r2+2r∂u(r,t)∂r]+f(r),t∈(0,1),0<r<1,u(r,0)=0,u(1,t)=0. |
Since the diffusivity a(t) is taken to be a constant, the specific representation of the eigenvalues, eigenfunctions and the regularization solution could be calculated as follows
kn=∫T0e−λn∫Tτa(t)dtdτ=1−e−aλnTaλn=1−e−a(nπ)2a(nπ)2, |
and the eigenfunctions
φn(r)=√2sin(nπr). |
Substituting the eigenfunctions gives a formula to compute the coefficients
⟨rg(r),φn(r)⟩=√2∫10rg(r)sin(nπr)dr. |
Note that the right hand side is essentially the sine transform of the function rg(r), which can be efficiently implemented by using a version of the fast Fourier transform for real functions [40]. Based on the explicit expressions for the eigensystems, the regularization solution of WTRM could be calculated as follows
fδ,μ(r)=2r∞∑n=1kαnkα+1n+μ[∫10rgδ(r)sin(nπr)dr]sin(nπr),12<α≤1. | (6.1) |
the regularization solution of FLRM could be calculated as follows
fδ,m(r)=2r∞∑n=1[1−(1−βk2n)m]αk−1n[∫10rgδ(r)sin(nπr)dr]sin(nπr),12<α≤1. | (6.2) |
In practical computation, we take the first n=40 terms of the sum (6.1) and (6.2).
Example 6.1. Consider a smooth exact solution:
f1(r)=100(1−r2), | (6.3) |
in interval [0,1].
Figures 1–3 show the numerical results of Example 6.1. Figure 1 shows the solution of forward problem and the unperturbed data function g(r). Figure 2 shows the comparison for the regularization parameter chosen by both the a-priori WTRM and the a-posteriori WTRM with respect to different noise level. Figure 3 shows the comparison for the regularization parameter chosen by both the a-priori FLRM and the a-posteriori FLRM with respect to different noise level. Table 1(a) and 1(b) show the relative error between the exact solution and the approximate solution calculated by the WTRM and the FLRM, respectively.
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0444 | 0.1199 | (b) | REpriori | 0.0438 | 0.1178 |
REposterior | 0.0216 | 0.0386 | REposterior | 0.0117 | 0.0606 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
From the numerical results, the approximate solution calculated by the a-posterior method is better than the a-prior method, and the relative error of the a-posterior calculation is smaller than that of the a-priori parameter choice. As the noise level increases, the numerical performance deteriorates.
Example 6.2. Consider the oscillating source function
f2(r)=2[1+cos(3πr)], | (6.4) |
in interval [0,1].
The numerical experiments of Example 6.2 is similar to those of Example 6.1. The result of Example 6.2 is shown in Figures 4–6 and Table 2.
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1940 | 0.2912 | (b) | REpriori | 0.1077 | 0.3394 |
REposterior | 0.1356 | 0.2306 | REposterior | 0.0411 | 0.1670 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
Example 6.3. Consider the nonsmooth source function:
f3(r)={5,0<r≤0.5,80(1−r)4,0.5<r<1. | (6.5) |
In this example, we compare the numerical results of two fractional regularization methods and classical regularization methods under the same parameter choice. Figure 7 shows the solution of forward problem and the unperturbed data function g(r) of Example 6.3. Figure 8 and Table 3 show the comparison of the numerical results of the weighted fractional Tikhonov regularization method and the classical Tikhonov regularization method. The result shows that the weighted fractional Tikhonov method outperforms the classical Tikhonov method under the same parameters, and the classic Tikhonov regularized solution oversmooths.
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1124 | 0.1475 | (b) | REpriori | 0.1194 | 0.1607 |
REposterior | 0.0499 | 0.0849 | REposterior | 0.1162 | 0.1047 | ||
(a) Relative error of weighted fractional Tikhonov method (α=0.65); | |||||||
(b) Relative error of classic Tikhonov method (α=1). |
Figure 9 and Table 4 show the comparison of the numerical results of the fractional Landweber regularization method and the classical Landweber regularization method. We can see that the fractional Landweber method provides better numerical result than the classical Landweber method under the same iterative steps.
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0453 | 0.1123 | (b) | REpriori | 0.0422 | 0.1220 |
REposterior | 0.0699 | 0.0953 | REposterior | 0.0769 | 0.0981 | ||
(a) Relative error of fractional Landweber method (α=0.8); | |||||||
(b) Relative error of classic Landweber method (α=1). |
As we have seen, the fractional methods include the classical method by introducing a new fractional parameter. We have proved the error estimates for the fractional regularization methods under the the a-priori parameter choice rule and the Morozov's parameter choice rule. The error estimates are order-optimal. This shows that in theory the fractional regularization methods are not inferior to the classical regularization method. Numerical results are also displays that the fractional methods can overcome the disadvantage of over-smoothing of the classical methods. The future research will be generalize the fractional regularization methods to some fractional differential equations with important application background.
This research is partially supported by the Fundamental Research Funds for the Central Universities, No. lzujbky-2020-12. The author is very grateful to the reviewers for their constructive comments and valuable suggestions, which greatly improved the quality of our papers.
All authors declare no conflict of interest in this paper.
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δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0444 | 0.1199 | (b) | REpriori | 0.0438 | 0.1178 |
REposterior | 0.0216 | 0.0386 | REposterior | 0.0117 | 0.0606 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1940 | 0.2912 | (b) | REpriori | 0.1077 | 0.3394 |
REposterior | 0.1356 | 0.2306 | REposterior | 0.0411 | 0.1670 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1124 | 0.1475 | (b) | REpriori | 0.1194 | 0.1607 |
REposterior | 0.0499 | 0.0849 | REposterior | 0.1162 | 0.1047 | ||
(a) Relative error of weighted fractional Tikhonov method (α=0.65); | |||||||
(b) Relative error of classic Tikhonov method (α=1). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0453 | 0.1123 | (b) | REpriori | 0.0422 | 0.1220 |
REposterior | 0.0699 | 0.0953 | REposterior | 0.0769 | 0.0981 | ||
(a) Relative error of fractional Landweber method (α=0.8); | |||||||
(b) Relative error of classic Landweber method (α=1). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0444 | 0.1199 | (b) | REpriori | 0.0438 | 0.1178 |
REposterior | 0.0216 | 0.0386 | REposterior | 0.0117 | 0.0606 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1940 | 0.2912 | (b) | REpriori | 0.1077 | 0.3394 |
REposterior | 0.1356 | 0.2306 | REposterior | 0.0411 | 0.1670 | ||
Relative error of WTRM (α=0.65); | Relative error of FLRM (α=0.8). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.1124 | 0.1475 | (b) | REpriori | 0.1194 | 0.1607 |
REposterior | 0.0499 | 0.0849 | REposterior | 0.1162 | 0.1047 | ||
(a) Relative error of weighted fractional Tikhonov method (α=0.65); | |||||||
(b) Relative error of classic Tikhonov method (α=1). |
δ | 0.01 | 0.05 | δ | 0.01 | 0.05 | ||
(a) | REpriori | 0.0453 | 0.1123 | (b) | REpriori | 0.0422 | 0.1220 |
REposterior | 0.0699 | 0.0953 | REposterior | 0.0769 | 0.0981 | ||
(a) Relative error of fractional Landweber method (α=0.8); | |||||||
(b) Relative error of classic Landweber method (α=1). |