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Strain-induced packing transition of Ih Cun@Ag55-n(n = 0, 1, 13, 43) clusters from atomic simulations

  • Strain is of significance in packing transition, but the key structural information for metal nanoclusters is still limited. Atomic simulations using molecular dynamics (MD) were performed to explore the microscopic details of atomic packing transition in four icosahedral (Ih) Cun@Ag55-n clusters without or with different number of Cu core atoms. Analytical tools were used to demonstrate the packing transition including internal energy per atom, shape factor, pair distribution functions, and atomic stress as well as cross-sectional images. The simulation results showed the differences of strain distribution between the surface and interior regions of these clusters at elevated temperature, which affected the transition temperatures of these four clusters. The increasing temperature resulted in strong tensile strain in the surfaces and Cu/Ag interfaces, which decreased the packing transition from Ih configuration as well as the shape changes.

    Citation: Jinhan Liu, Lin Zhang. Strain-induced packing transition of Ih Cun@Ag55-n(n = 0, 1, 13, 43) clusters from atomic simulations[J]. Mathematical Biosciences and Engineering, 2020, 17(6): 6390-6400. doi: 10.3934/mbe.2020336

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  • Strain is of significance in packing transition, but the key structural information for metal nanoclusters is still limited. Atomic simulations using molecular dynamics (MD) were performed to explore the microscopic details of atomic packing transition in four icosahedral (Ih) Cun@Ag55-n clusters without or with different number of Cu core atoms. Analytical tools were used to demonstrate the packing transition including internal energy per atom, shape factor, pair distribution functions, and atomic stress as well as cross-sectional images. The simulation results showed the differences of strain distribution between the surface and interior regions of these clusters at elevated temperature, which affected the transition temperatures of these four clusters. The increasing temperature resulted in strong tensile strain in the surfaces and Cu/Ag interfaces, which decreased the packing transition from Ih configuration as well as the shape changes.


    The Euler numbers En are defined by the exponential generating function

    1+n1Enxnn!=tanx+secx.

    This is the sequence A000111 in [20]. In 1877 Seidel [19] defined the triangular array (En,k) by the recurrence

    En,k=En,k1+En1,n+1k(nk2) (1)

    with E1,1=1, En,1=0 (n2), and proved that En=kEn,k, i.e., the Euler number En is the sum of the entries of the n-th row of the following triangle:

    E1,1E2,1E2,2E3,3E3,2E3,1E4,1E4,2E4,3E4,4=1011100122 (2)

    The first few values of En,k are given in Table 1.

    Table 1.  The Entringer numbers En,k for 1kn7 and Euler numbers En=nk=1En,k.
    nk 1 2 3 4 5 6 7 En
    1 1 1
    2 0 1 1
    3 0 1 1 2
    4 0 1 2 2 5
    5 0 2 4 5 5 16
    6 0 5 10 14 16 16 61
    7 0 16 32 46 56 61 61 271

     | Show Table
    DownLoad: CSV

    André [1] showed in 1879 that the Euler number En enumerates the alternating permutations of [n]:={1,2,,n}, i.e., the permutations σ1σ2σn of 12n such that σ1>σ2<σ3>σ4<. Let DUn be the set of (down-up) alternating permutations of [n]. For example,

    DU4={2143,3142,3241,4132,4231}.

    In 1933 Kempener [14] used the boustrophedon algorithm (2) to enumerate alternating permutations without refering to Euler numbers. Since Entringer [7] first found the combinatorial interpretation of Kempener's table (En,k) in terms of André's model for Euler numbers, the numbers En,k are then called Entringer numbers.

    Theorem 1 (Entringer). The number of the (down-up) alternating permutations of [n] with first entry k is En,k, i.e., En,k=#DUn,k, where

    DUn,k:={σDUn:σ1=k}.

    According to Foata-Schützenberger [9] a sequence of sets (Xn) is called an André complex if the cardinality of Xn is equal to En for n1. Several other André complexs were introduced also in [9] such as André permutations of first and second kinds, André trees or increasing 1-2 trees and Rodica Simion and Sheila Sundaram [23] discovered the Simsun permutations; see Section 2. A sequence of sets (Xn,k) is called an Entringer family if the cardinality of Xn,k is equal to En,k for 1kn. During the last two decades of the twentieth century, Poupard worked out several Entringer families in a series of papers [15,16,17]. Stanley [22,Conjecture 3.1] and Hetyei [12] introduced more Entringer families by refining of Purtill's result [18,Theorem 6.1] about the cd-index of André and Simsun permutations with fixed last letter.

    The Springer numbers Sn are defined by the exponential generating function [21]

    1+n1Snxnn!=1cosxsinx.

    Arnold [2,p.11] showed in 1992 that Sn enumerates a signed-permutation analogue of the alternating permutations. Recall that a signed permutation of [n] is a sequence π=(π1,,πn) of elements of [±n]:={1,,n}{1,,n} such that |π|=(|π1|,,|πn|) is a permutation of [n]. We write Bn for the set of all signed permutations of [n]. Clearly the cardinality of Bn is 2nn!. An (down-up) alternating permutation of type Bn is a signed permutation πBn such that π1>π2<π3>π4 and a snake of type Bn is an alternating permutation of type Bn starting with a positive entry. Let DUn(B) be the set of (down-up) alternating permutations of type Bn and Sn the set of snakes of type Bn. Clearly the cardinality of DUn(B) is 2nEn. Arnold [2] showed that the Springer number Sn enumerates the snakes of type Bn. For example, the 11 snakes of S3 are as follows:

    1ˉ23,1ˉ32,1ˉ3ˉ2,213,2ˉ13,2ˉ31,2ˉ3ˉ1,312,3ˉ12,3ˉ21,3ˉ2ˉ1,

    where we write ˉk for k. Arnold [2] introduced the following pair of triangles to compute the Springer numbers:

    S1,1S2,2S2,1S3,3S3,2S3,1S4,4S4,3S4,2S4,112102316161411S1,1S2,1S2,2S3,1S3,2S3,3S4,1S4,2S4,3S4,411034411840

    where Sn,k is defined by S1,1=S1,1=1, Sn,n=0 (n2), and the recurrence

    Sn,k={Sn,k1+Sn1,k+1if nk>1,Sn,1if n>k=1,Sn,k1+Sn1,kif 1k>n. (3)

    Theorem 2 (Arnold). For all integers 1kn, the number of the snakes of type Bn starting with k is Sn,k, i.e., Sn,k=#Sn,k with Sn=k>0#Sn,k, where

    Sn,k:={σSn:σ1=k}.

    Moreover, for all integers nk1, it holds that

    Sn,k=#{σDUn(B):σ1=k}.

    Similarly, the numbers Sn,k are called Arnold numbers and a sequence of sets (Xn,k) is called an Arnold family if the cardinality of Xn,k is equal to Sn,k for 1|k|n. The first values of Arnold and Springer numbers are given in Table 2. The aim of this paper is to provide some new Entringer families and new Arnold families by refining known combinatorial models for Euler and Springer numbers. To this end we shall build bijections between these new Entringer (resp. Arnold) families with the known ones. We refer the reader to the more recent papers [23,11,4,13,8] related to the combinatorics of Euler numbers and Springer numbers.

    Table 2.  The Arnold numbers Sn,k for 1|k|n6 and Springer numbers Sn=nk=1Sn,k.
    nk -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 Sn
    1 1 1 1
    2 0 1 1 2 3
    3 0 2 3 3 4 4 11
    4 0 4 8 11 11 14 16 16 57
    5 0 16 32 46 57 57 68 76 80 80 361
    6 0 80 160 236 304 361 361 418 464 496 512 512 2763

     | Show Table
    DownLoad: CSV

    This paper is organized as follows. In Section 2, we shall give the necessary definitions and present our main results. The proof of our theorems will be given in Sections 3-4. In Section 5, we shall give more insightful description of two important bijections. More precisely, Chuang et al.'s constructed a ϕ:Tn+1RSn in [4], we show that ϕ can be factorized as the compositions of two of our simpler bijections, and then give a direct description of Gelineau et al.'s bijection ψ:DUnTn in [11].

    Let V be a finite subset of N. An increasing 1-2 tree on V is a vertex labeled rooted tree with at most two (upward) branchings at any vertex and vertex labels in increasing order on any (upward) path from the root, see Figure 2. In what follows, when one draws an increasing 1-2 tree, let's designate the left child if its parent has a unique child or it is smaller than the other sibling and the others, except of the root, are designate the right child.

    Figure 1.  increasing 1-2 trees on [4].
    Figure 2.  Sixteen type B increasing 1-2 trees on [3].

    For each vertex v of a binary tree, by exchanging the left and right subtrees of v, we obtain another binary tree. This operation is called a flip. If two binary trees can be connected by a sequence of flips, we say that these two trees are flip equivalent. Since flip equivalence is obviously an equivalence relation, we are able to define the equivalence classes of binary trees, which are corresponding to increasing 1-2 trees; see [23,Section 3.2] in detail.

    Definition 3. Given an increasing 1-2 tree T, the minimal path of T is the unique sequence (v1,,v) of vertices where v1 is the root, vk+1 is the left child of vk (1k<), and v is a leaf. The vertex v is called the minimal leaf of T and denoted by Leaf(T). Similarly, the unique path (v1,,v) from v1=v to a leaf v of T is called the maximal path from v if vk+1 is the right child of vk for 1k<.

    Let TV be the set of increasing 1-2 trees on V with Tn:=T[n] and

    Tn,k={TTn:Leaf(T)=k}.

    Donaghey [5] (see also [3]) proved bijectively that the Euler number En enumerates the binary increasing trees in Tn and Poupard [15] showed that the sequence (Tn,k) is an Entringer family. In a previous work Gelineau et al. [11] proved bijectively Poupard's result by establishing a bijection between DUn and Tn.

    Theorem 4 (Gelineau-Shin-Zeng). There is an explicit bijection ψ:DUnTn such that

    Leaf(ψ(σ))=First(σ)

    for all σDUn, where First(σ) is the first entry of σ.

    Let Sn be the group of permutations on [n]. For a permutation σ=σ1σnSn, a descent (resp. ascent) of σ is a pair (σi,σi+1) with σi>σi+1 (resp. σi<σi+1) and 1in1, a double descent of σ is a triple (σi,σi+1,σi+2) with σi>σi+1>σi+2 and 1in2, and a valley of σ is a triple (σi,σi+1,σi+2) with σi>σi+1<σi+2 and 1in2.

    Hetyei [12,Definition 4] defined recursively André permutation of second kind if it is empty or satisfies the following:

    (ⅰ) σ has no double descents.

    (ⅱ) (σn1,σn) is not descent, i.e., σn1<σn.

    (ⅲ) For all 2in1, if (σi1,σi,σi+1) is a valley of σ, then the minimum letter of w2 is larger than the minimum letter of w4 for the σi-factorization (w1,w2,σi,w4,w5) of σ, where the word w1w2σiw4w5 is equal to σ and w2 and w4 are maximal consecutive subwords of σ satisfies its all letters are greater than σi.

    It is known that the above definition for André permutation of second kind is simply equivalent to the following definition. Let σ[k] denote the subword of σ consisting of 1,,k in the order they appear in σ.

    Definition 5. A permutation σSn is called an André permutation if σ[k] has no double descents and ends with an ascent for all 1kn.

    For example, the permutation σ=43512 is not André since the subword σ[4]=4312 contains a double descent (4,3,1), while the permutation τ=31245 is André since there is no double descent in the subwords:

    τ[1]=1,τ[2]=12,τ[3]=312,τ[4]=3124,τ[5]=31245.

    Foata and Schützenberger [10] proved that the Euler number En enumerates the André permutations in Sn. Let An be the set of André permutations in Sn. For example,

    A4={1234,1423,3124,3412,4123}.

    Remark. Foata and Schützenberger in [10] introduced augmented André permutation is a permutation σSn, if σ has no double descents, σn=n, and, for 1<j<kn satisfying

    σj1=max{σj1,σj,σk1,σk}andσk=min{σj1,σj,σk1,σk},

    there exists such that j<<k and σ<σk.

    Definition 6. A permutation σSn is called a Simsun permutation if σ[k] has no double descents for all 1kn.

    By definition, an André permutations is always a Simsun permutation, but the reverse is not true. For example, the permutation σ=25134 is Simsun but not André, because τ[2]=21 ends with an descent:

    τ[1]=1,τ[2]=21,τ[3]=213,τ[4]=2134,τ[5]=25134.

    Let RSn be the set of Simsun permutations in Sn. For example,

    RS3={123,132,213,231,312}.

    As for DUn,k, we define two similar refinements of André permutations and Simsun permutations as

    An,k:={σAn:σn=k},RSn,k:={σRSn:σn=k}.

    Some examples are shown in Table 3.

    Table 3.  The sets DU4,k, A4,k, and RS3,k1 for 2k4.
    k DU4,k A4,k RS3,k1
    2 {2143} {3412} {231}
    3 {3142,3241} {1423,4123} {132,312}
    4 {4132,4231} {1234,3124} {123,213}

     | Show Table
    DownLoad: CSV

    Foata and Han [8,Theorem 1 (ⅲ)] proved that An,k is an Entringer family by constructing a bijection between DUn,k and An,k. We shall give an easier proof of their result by constructing a simpler bijection ω between Tn,k and An,k. Of course, combining ψ (cf. Theorem 4) and ω we obtain another bijection between DUn,k and An,k.

    Theorem 7. For positive integer n1, there is a bijection ω:TnAn such that

    Leaf(T)=Last(ω(T)) (4)

    for all TTn, where Last(σ) is the last entry of σ. In words, for all 1kn, the mapping ω is a bijection from Tn,k onto An,k.

    Whereas one can easily show that the cardinality #DUn,k of (down-up) alternating permutations of length n with first entry k satisfies (1), it seems hard to show that the cardinality #An,k of André permutations of length n with last entry k or the cardinality #RSn1,k1 of Simsun permutations of length n1 with last entry k1 does. Thus, in order to show (4) and (6), we shall construct a bijection between DUn,k, An,k, RSn1,k1, and other known Entringer families in [11].

    Stanley [22,Conjecture 3.1] conjectured a refinement of Purtill's result [18,Theorem 6.1] about the cd-index of André and Simsun permutations with fixed last letter. In this conjecture, he mentioned three kinds of André permutations: (ⅰ) Augmented André permutations in Remark 2, (ⅱ) André permutations in Definition 5, and (ⅲ) augmented Sundaram permutations, where the third corresponds to Simsun permutations in Definition 6 by removing last letter. Hetyei [12] proved the conjecture for the second and the third by verifying that both sides satisfy the same recurrence. In particular, he proves the following result.

    Theorem 8 (Hetyei). For all 1kn, we have that two cardinalities of An,k and RSn1,k1 are same, that is,

    #An,k=#RSn1,k1. (5)

    In the next theorem, we give a bijective proof of the conjecture of Stanley by constructing an explicit bijection.

    Theorem 9. For positive integer n1, there is a bijection φ:AnRSn1 such that

    Last(σ)1=Last(φ(σ)) (6)

    for all σAn. In words, the mapping φ is a bijection from An,k onto RSn1,k1. Moveover, the bijection φ preserves the cd-index of André and Simsun permutation.

    Given a permutation σBn, denote σ[k] the subword of σ consisting of k smallest entries in the order they appear in σ. A signed André permutation of [n] is a permutation σBn such that σ[k] has no double descents and ends with an ascent for all 1kn. Let A(B)n be the set of signed André permutations of [n] and A(B)n,k be the set of signed André permutations σ in A(B)n ending with entry k. For example, the permutation σ=2ˉ4ˉ135 is Andŕe due to

    σ[1]=ˉ4,σ[2]=ˉ4ˉ1,σ[3]=2ˉ4ˉ1,σ[4]=2ˉ4ˉ13,σ[5]=2ˉ4ˉ135.

    Some examples of A(B)3,k are shown in Table 4.

    Table 4.  The sets S3,k, A(B)3,k, A(H)4,5k, and RS(B)3,4k for 1k3.
    k S3,k A(B)3,k A(H)4,5k RS(B)3,4k
    1 {1ˉ23,1ˉ32,1ˉ3ˉ2} {3ˉ21,ˉ3ˉ21,2ˉ31} {1234,3124,ˉ3124} {123,213,ˉ213}
    2 {213,2ˉ13,2ˉ31,2ˉ3ˉ1} {312,ˉ312,3ˉ12,ˉ3ˉ12} {1423,1ˉ423,4123,ˉ4123} {132,1ˉ32,312,ˉ312}
    3 {312,3ˉ12,3ˉ21,3ˉ2ˉ1} {ˉ213,ˉ2ˉ13,123,ˉ123} {3412,ˉ3412,3ˉ412,ˉ3ˉ412} {231,ˉ231,2ˉ31,ˉ2ˉ31}

     | Show Table
    DownLoad: CSV

    Definition 10. A type B increasing 1-2 tree on [n] is a binary tree with n signed labels in {±1,±2,,±n} such that the absolute values of signed labels are distinct and any vertex is greater than its children.

    For example, all type B increasing 1-2 trees on [3] are given in Figure 2. Let T(B)n be the set of type B increasing 1-2 trees on n vertices and T(B)n,k be the set of trees T in T(B)n with leaf k as the end of minimal path. Clearly we have T(B)n=|k|>0T(B)n,k.

    Our second aim is to show that these two refinements are new Arnold families. Recall that the sequence Sn,k is an Arnold family for 1|k|n as

    Sn,k:={σDUn(B):σ1=k}.

    Theorem 11. For all 1|k|n, there are two bijections

    ψB:Sn,kT(B)n,k, (7)
    ωB:T(B)n,kA(B)n,k. (8)

    Thus, for all 1|k|n,

    Sn,k=#A(B)n,k=#T(B)n,k. (9)

    In particular, the two sequences A(B)n,k and T(B)n,k are Arnold families for 1|k|n.

    Hetyei[12,Definition 8] defined another class of signed André permutations.

    Definition 12 (Hetyei). A signed André permutation is a pair (ε,π), where π is an André permutation such that ε(i)=1 if πi=min{πi,πi+1,,πn}.

    We write A(H)n (resp. A(H)n,k) for the set of the signed André permutations (resp. ending with entry k) in Bn. Some examples of A(H)4,k are shown in Table 4. We have the following conjecture.

    Conjecture 13. For all 1kn, we have

    Sn,k=#A(H)n+1,n+2k.

    Since the last entry of any permutation in the family A(H)n is always positive, even if Conjecture 13 is true, it covers only the half of Table 2. Now we define signed Simsun permutations corresponding to Heytei's signed André permutations.

    Definition 14. A permutation π in Bn is a signed Simsun permutation if |π1||π2||πn| is a Simsun permutation and πi>0 for all |πi|=min{|πi|,|πi+1|,,|πn|}.

    Let RS(B)n be the set of signed Simsun permutations in Bn and RS(B)n,k the set of signed Simsun permutations in RS(B)n with last entry k. Some examples of RS(B)3,k are shown in Table 4.

    Theorem 15. For positive integer n1, there is a bijection φ(B):A(H)nRS(B)n1 such that

    Last(σ)1=Last(φ(B)(σ)) (10)

    for all σA(H)n. In words, the mapping φ(B) is a bijection from A(H)n,k onto RS(B)n1,k1.

    Remark. Ehrenborg and Readdy [6,Section 7] gave a different definition of signed Simsun permutation as follows: A signed permutation σ of length n is a Simsun permutation if σ[k] have no double descents for all 1kn, where σ[k] is obtained by removing the (nk) entries ±(k+1),,±n from σ. Beacuse all eight signed permutations of length 2

    12,21,ˉ12,2ˉ1,1ˉ2,ˉ21,ˉ1ˉ2,ˉ2ˉ1

    are Simsun permutations, we note that it is not an Arnold family.

    First of all, we prove Theorem 7, in order to show that (An,k)1kn is a Entringer family, that is, En,k=#An,k. We construct a bijection ω between Tn,k and An,k in Section 3.1. Hence the map ωψ is a bijection from the set DUn,k of (down-up) alternating permutations with first entry k to the set An,k of André permutations with last entry k. Also, in order to show that (RSn1,k1)1kn is a Entringer family, that is En,k=#RSn1,k1, we construct a bijection between An,k and RSn1,k1 in Section 3.2 and then two sets An,k and RSn1,k1 have the same cardinality.

    Given TTn,k, write down the word σ of the vertices of the tree T in inorder, namely, for any vertex v in T, the left child of v and its descendants precede the vertex v and the vertex v precede the right child of v and its descendants. Since T is an increasing tree, we can recover T from σ by finding minimum in subwords in σ successively. The word σ has no double ascents because no vertex in T has only a right child. The leaf p(T) of the minimal path of T is σ1 and the parent of σ1 is σ2, so σ starts with a decent, that is, σ1>σ2. Similarly, since the subgraph of T consisting of 1,,k, for any k, is also well-defined an increasing 1-2 subtree, the subwords of σ consisting of 1,,k has also no double ascents and starts with a decent. Thus the word σR, which is the reverse word of σ, is an André permutation of n and let ω(T)=σR.

    For example, if the tree T is given by the following figure with a corresponding word σ,

    12,21,ˉ12,2ˉ1,1ˉ2,ˉ21,ˉ1ˉ2,ˉ2ˉ1

    then ω(T)=σR=684512937 as reading reversely vertices of T in inorder.

    Given σ:=σ1σnAn,k, let i1<<i be the positions of the right-to-left minima of σ. Clearly σi1=1, i1=n1 and i=n with σn=k. Let φ(σ)=π, where π=π1π2πn1 is defined by

    πi={σi1ifi{i1,,i},σik1ifi=ik1fork=2,,. (11)

    We show that πRSn1,k1. Suppose π[i] has a double descent for some 1in. There exists a triple (a,b,c) such that 1a<b<cn, iπa>πb>πc, and πa+1,,πb1, πb+1,,πc1 are greater than i, which yields πc=min{πj:ajc}. Then πc could be a right-to-left minimum in π and the others πa,πa+1,,πc1 are not in π. As φ(σ)=π, we have

    σa=πa+1,σa+1=πa+1+1,,σc1=πc1+1, and σcπc+1.

    Hence a triple (σa,σb,σc) is a double descent in σ[i], which contradicts that σ is an André permutation. As this procedure is clearly reversible, the mapping φ is a bijection.

    Consider the running example σ=684512937. The right-to-left minimums of σ are 1, 2, 3, 7. So after removing 1 from σ, the entries 2, 3, 7 are moved to the positions of 1, 2, 3, respectively, and we get the permutation ˆπ=68452397, and then φ(σ)=π=57341286, which is a Simsun permutation of length 8 with last entry 6.

    Remark. Considering the bijection ψ in Theorem 4, the map φωψ is a bijection from the set of (down-up) alternating permutations of length n with first entry k to the set of Simsun permutations of length n1 with last entry k1. Namely we have the diagram in Figure 3. For example, if τ=739154826DU9,7, then

    ψ(τ)=TT9,7,ω(T)=σ=684512937A9,7,φ(σ)=π=57341286RS8,6,
    Figure 3.  Bijections between Entringer families and Arnold families.

    where T is the increasing 1-2 tree given in Section 3.1.

    One can extend the above mapping φ defined on An to a mapping φ(B) on A(H)n. It is also bijective between A(H)n,k and RS(B)n1,k1, but the description of φ(B) and a proof of bijectivity are omitted, because it is very similar in Section 3.2.

    Remark. This bijection preserves the cd-indices between André permutations and Simsun permutations. The variation of a permutation π=π1πn is given by ab-monomial u1un1 such that ui=a if πi<πi+1 and ui=b if πi>πi+1. The reduced variation of André permuation is defined by replacing each ba with d and then replacing each remaining a by c. For example, the variation and reduced variation of André permutation σ=684512937 is

    ababaaba=cddcd.

    For the cd-index of a Simsun permutation σ, we consider augmented Simsun permutation by adding σ(0)=0. Here, the reduced variation of augmented Simsun permuations is defined by replacing each ab with d and then replacing each remaining a by c. So the variation and reduced variation of augmented Simsun permutation 057341286 by adding 0 to φ(σ)=57341286 is

    aababaab=cddcd.

    Given a σSn,k, there is a unique order-preserving map πσ, say just π, from {σ1,,σn} to [n]. In other words, π replaces the i-th smallest entry in σ by i. The permutation τ=πσ belongs to DUn,k and ψ(τ)=ψ(πσ) in Tn,k. Then π1(ψ(τ)) means the tree with vertex labelings {σ1,,σn} instead of [n] and it should belong to T(B)n,k. So we construct the bijection ψB from Sn,k to T(B)n,k by

    ψB(σ)=π1(ψ(πσ))

    through the unique order-preserving map π. Hence, it yields (7).

    For example, in the case of σ=6ˉ39ˉ82ˉ17ˉ45, the order-preserving map πσ is

    \pi = {\bar{8}\bar{4}\bar{3}\bar{1}25679 \choose 123456789}.

    So we have \tau = \pi \sigma = 739154826 and \psi(\tau) = \psi(\pi \sigma) and \psi^{B}(\sigma) = \pi^{-1}(\psi(\pi \sigma)) are illustrated as

    \pi = {\bar{8}\bar{4}\bar{3}\bar{1}25679 \choose 123456789}.

    In Subsection 3.1, we define the bijection \omega from {{\mathcal{T}}_{n,k}} to {{\mathcal{A}}_{n,k}} . Given a tree T in {{{\mathcal{T}}^{(B)}}_{n,k}} , there is a unique order-preserving map \pi_T , say just \pi , from V(T) to [n] . In other words, \pi replaces the i -th smallest V(T) by i . After relabeling on vertices of T by \pi , we obtain the tree \pi(T) which belongs to {{\mathcal{T}}_{n,k}} and \omega(\pi(T)) is in {{\mathcal{A}}_{n,k}} . Then \pi^{-1}(\omega(\pi(T))) should belong to {{{\mathcal{A}}^{(B)}}_{n,k}} . So we construct the bijection \omega^{B} from {{{\mathcal{T}}^{(B)}}_{n,k}} to {{{\mathcal{A}}^{(B)}}_{n,k}} by

    \omega^{B}(T) = \pi^{-1}(\omega(\pi(T)))

    through the unique order-preserving map \pi . Such the map \omega^{B} can be described simply, as same as \omega , by reading reversely vertices of T in inorder. Hence, it yields (8).

    For example, in the case of T illustrated as

    \omega^{B}(T) = \pi^{-1}(\omega(\pi(T)))

    we obtain \omega^B(T) = \sigma^R = 57\bar{1}2\bar{8}\bar{4}9\bar{3}6 by reading reversely vertices of T in inorder. So bijections for type A and type B commute in the diagram of Figure 3.

    We summarize four interpretations for Entringer numbers E_{4,k} , k \in \left\{{2,3,4}\right\} in Table 5 and left three interpretations for Arnold number S_{3,k} , k \in \left\{{1,2,3}\right\} in Table 6. In every column, the corresponding elements are described via the different bijections mentioned in the paper.

    Table 5.  Three bijections between Entringer families with n = 4 and 2\le k \le 4 .
    k \tau \in {\mathcal{DU} }_{4,k} \psi(\tau) \in {\mathcal{T}}_{4,k} \omega(\psi(\tau)) \in {\mathcal{A}}_{4,k} \varphi(\omega(\psi(\tau))) \in {\mathcal{RS}}_{3,k-1}
    2 2143 3412 231
    3 3241 1423 132
    3142 4123 312
    4 4231 1234 123
    4132 3124 213

     | Show Table
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    Table 6.  Three bijections between Arnold families with n = 3 and 1\le k \le 3 .
    \tau \in {\mathcal{S}}_{3,k} \psi^{(B)}(\tau) \in {{\mathcal{T}}^{(B)}}_{3,k} \omega^{(B)}(\psi^{(B)}(\tau)) \in {{\mathcal{A}}^{(B)}}_{3,k} k \sigma \in {{\mathcal{A}}^{(H)}}_{4,5-k} \varphi^{(B)}(\sigma) \in {{\mathcal{RS}}^{(B)}}_{3,4-k}
    1\bar{2}3 3\bar{2}1 1 1234 123
    1\bar{3}2 2\bar{3}1 3124 213
    1\bar{3}\bar{2} \bar{3}\bar{2}1 \bar{3}124 \bar{2}13
    213 312 2 1423 132
    2\bar{1}3 3\bar{1}2 1\bar{4}23 1\bar{3}2
    2\bar{3}1 \bar{3}12 4123 312
    2\bar{3}\bar{1} \bar{3}\bar{1}2 \bar{4}123 \bar{3}12
    312 123 3 3412 231
    3\bar{1}2 \bar{1}23 \bar{3}412 \bar{2}31
    3\bar{2}1 \bar{2}13 3\bar{4}12 2\bar{3}1
    3\bar{2}\bar{1} \bar{2}\bar{1}3 \bar{3}\bar{4}12 \bar{2}\bar{3}1

     | Show Table
    DownLoad: CSV

    In 2012, Chuang et al. [4] construct a bijection \phi: {\mathcal{T}}_{n+1} \to {\mathcal{RS}}_{n} . If x has only one child then it is the right child of x . They described the bijection \phi between increasing 1-2 trees and Simsun permutations using the following algorithm.

    Algorithm A.

    (A1) If T consists of the root vertex then T is associated with an empty word.

    (A2) Otherwise, the word \rho(T) is defined inductively by the factorization

    \rho(T) = \omega\cdot \rho(T'),

    where the subword \omega and the subtree T' are determined as follows.

    (a) If the root of T has only one child x then let \omega = x (consisting of a single letter x ), let T' = \tau(x) (i.e, obtained from T by deleting the root of T ), and relabel the vertex x by 1 .

    (b) If the root of T has two children u , v with u>v then traverse the right subtree \tau(u) reversely in inorder, put down the word \omega of the vertices of \tau(u) and let T' = T-\tau(u) .

    As deleted only 1 in \rho(T) , every permutation \rho(T) = a_1 a_2\cdots a_n is a Simsun permutation on \left\{{2, 3, \dots, n+1}\right\} . Thus we get a Simsun permutation \phi(T) = b_1 b_2\cdots b_n on [n] with b_i = a_i - 1 for all 1\le i \le n .

    Remark. Originally, in [4], the increasing 1-2 trees on n vertices are labeled with 0,1, \ldots, n-1 instead of 1,2, \ldots, n and was drawn in a canonical form such that if a vertex x has two children u , v with u > v then u is the left child, v is the right child.

    Theorem 16. The bijection \phi : {{\mathcal{T}}_{n,k}} \to {{\mathcal{RS}}_{n-1,k-1}} is the composition of \varphi and \omega ; i.e., \phi = \varphi \circ \omega .

    Proof. Suppose that we let \omega be the root of T without relabeling the vertex x in (A2a), it is obvious that Algorithm A follow the bijeciton \omega , i.e., reading the vertex of T reversely in inorder. So it is enough to show that the change in two rules of (A2a) follows \varphi .

    The root of T' becomes to the left-child of the root of T after (A2a) and the root of T' is same with the root of T in (A2b). So Algorihm A executes the step (A2a) only when a vertex on the minimal path of an original tree becomes the root of its subtree.

    To record the left-child x instead of the root 1 with relabeling the vertex x by 1 in each of (A2a)'s means to exchange x and 1 sequentially according to the minimal path from the root 1 of the original tree T .

    It is clear that all vertices in the minimal path in a tree become the right-to-left minimums in a permutation under \omega . So each right-to-left minimums is recorded in the position of the previous right-to-left minimums in a permutation obtained from a tree by \omega . Since all elements are decreased by 1 in the last step, it satisfies (11), then Algorithm A follows \varphi \circ \omega .

    The bijection \psi between {{{\mathcal{DU} }}_{n,k}} and {{\mathcal{T}}_{n,k}} was constructed as a composition of two bijections via the set \mathcal{ES}_{n,k} of encoding sequences in [11]. In this section, we just give directly another description of this bijection \psi from {{{\mathcal{DU} }}_{n,k}} to {{\mathcal{T}}_{n,k}} , which does not use encoding sequences.

    Given an increasing 1-2 tree T \in T_n , by convention, if a vertex x of T has two children u , v with u<v then u is the left child and v is the right child. By convention, if x has only one child then it is the left child of x .

    Algorithm B. Gelineau et al. described the bijection \psi between alternating permutations and increasing 1-2 trees using the following algorithm. Due to, for n = 1 or 2 ,

    \left|{{{\mathcal{DU} }}_n}\right| = \left|{{\mathcal{T}}_n}\right| = 1,

    we can define trivially \psi . For n\ge 3 , given \pi \in {{{\mathcal{DU} }}_{n,k}} ( k = \pi_1 ), we define the mapping \psi:{{{\mathcal{DU} }}_{n,k}} \to {{\mathcal{T}}_{n,k}} recursively as follows:

    (B1) If \pi_2 = k-1 , then define \pi' = \pi'_1\pi'_2 \dots \pi'_{n-2} \in {{\mathcal{DU} }}_{n-2,i-2} by deleting k-1 and k from \pi and relabeling by [n-2] where i> k , that is, for all 1\le j \le n-2

    \pi'_{j} = \begin{cases} \pi_{j+2}, &\text{if $\pi_{j+2} < k-1$,}\\ \pi_{j+2}-2, &\text{if $\pi_{j+2} > k$.} \end{cases}

    We get T' = \psi(\pi') \in {\mathcal{T}}_{n-2,i-2} . Relabel T' by \left\{{1,\dots,k-2,k+1,\dots,n}\right\} keeping in the order of labels, denoted by T'' . Let m be the smallest vertex greater than k in the minimal path of T'' and \ell the parent of m in T'' . Then insert a vertex k-1 in the middle of the edge (\ell,m) and graft k as the left-child of k-1 .

    \pi'_{j} = \begin{cases} \pi_{j+2}, &\text{if $\pi_{j+2} < k-1$,}\\ \pi_{j+2}-2, &\text{if $\pi_{j+2} > k$.} \end{cases}

    We get the tree T = \psi(\pi) \in {{\mathcal{T}}_{n,k}} .

    (B2) If \pi_2<k-1 , then define \pi' = (k-1\; k)\pi \in {{\mathcal{DU} }}_{n,k-1} by exchanging k-1 and k in \pi . We get T' = \psi(\pi') \in {\mathcal{T}}_{n,k-1} .

    (a) If k is a not sibling of k-1 in T' , then we get the tree T = \psi(\pi) \in {{\mathcal{T}}_{n,k}} exchanging the labels k-1 and k in T' .

    \pi'_{j} = \begin{cases} \pi_{j+2}, &\text{if $\pi_{j+2} < k-1$,}\\ \pi_{j+2}-2, &\text{if $\pi_{j+2} > k$.} \end{cases}

    (b) If k is a sibling of k-1 in T' , then we get the tree T = \psi(\pi) \in {{\mathcal{T}}_{n,k}} modifying as follows:

    \pi'_{j} = \begin{cases} \pi_{j+2}, &\text{if $\pi_{j+2} < k-1$,}\\ \pi_{j+2}-2, &\text{if $\pi_{j+2} > k$.} \end{cases}

    Algorithm C. We define another bijection \psi' : {{{\mathcal{DU} }}_{n,k}} \to {{\mathcal{T}}_{n,k}} by the following algorithm. Given \sigma = \sigma_1 \dots \sigma_n \in {{{\mathcal{DU} }}_{n,k}} , denote by d_i(\sigma) = (\sigma_{2i-1}, \sigma_{2i}) with \sigma_{2i-1} > \sigma_{2i} for 1 \le i \le m , where m = \lfloor (n+1)/2\rfloor , but d_{m}(\sigma) = (\sigma_{n}) if n is odd. We shall construct a series of trees T^{(m)}, \ldots, T^{(1)} by reading the pairs d_m(\sigma) , \ldots, d_1(\sigma) in this order.

    If n is odd, so d_{m} (\sigma) = (\sigma_n) , define T^{(m)} to be the tree with only root \sigma_n . If n is even, so d_{m}(\sigma) = (\sigma_{n-1}, \sigma_{n}) with \sigma_{n-1} > \sigma_{n} , define T^{(m)} to be the tree with the root \sigma_{n} and its left child \sigma_{n-1} . We note that the vertex \sigma_{2m-1} is the minimal leaf of T^{(m)} .

    For 1 \le i \le m-1 , assuming that we have a tree T^{(i+1)} of which the minimal leaf is \sigma_{2i+1} , read d_i(\sigma) = (\sigma_{2i-1}, \sigma_{2i}) . As \sigma_{2i} < \sigma_{2i+1} , the smallest vertex, say a^{(i)} , greater than \sigma_{2i} in the minimal path of T^{(i+1)} is well-defined. By removing all left edges from the increasing 1-2 tree T^{(i+1)} , we have several paths, each connected component of which is called a maximal path consisting only right edges.

    (C1) If a^{(i)} < \sigma_{2i-1} , the largest vertex, say b^{(i)} , less than \sigma_{2i-1} in the maximal path from a^{(i)} of T^{(i+1)} is well-defined. For some j\ge 1 , let (v_1, v_1, \ldots, v_j) be the path from a^{(i)} to b^{(i)} with v_1 = a^{(i)} and v_j = b^{(i)} . The vertices v_1, \dots, v_{j-1} should have left child u_1, \dots, u_{j-1} , but v_j may not, with

    \begin{align*} v_1 < u_1 < v_2 < u_2 < \dots < v_{j-1} < u_{j-1} < v_j \end{align*}

    Decomposing by the maximal path from a^{(i)} to b^{(i)} , we write S_1, \ldots, S_{j} the left-subtrees of the vertices v_1, \ldots, v_{j} and S_{j+1} the right-subtree of b^{(i)} . Since each of u_1, \dots, u_{j-1} lies on each of S_1, \dots, S_{j-1} , S_1, \dots, S_{j-1} should not be empty, but two trees S_{j} and S_{j+1} may be empty. Then, the tree T^{(i)} is obtained from T^{(i+1)} by the following operations:

    ● Graft \sigma_{2i} so that a^{(i)} is its left-child;

    ● Flip the tree at vertex a^{(i)} ;

    ● Transplant the trees S_1, \ldots, S_{j+1} as right-subtrees of the vertices \sigma_{2i} , a^{(i)} , v_2, \ldots, v_{j-1} , b^{(i)} ;

    ● Graft \sigma_{2i-1} as the left-child of b^{(i)} .

    We can illustrate the above transformation by

    \begin{align*} v_1 < u_1 < v_2 < u_2 < \dots < v_{j-1} < u_{j-1} < v_j \end{align*}

    (C2) If \sigma_{2i-1}<a^{(i)} , then b^{(i)} does not exist. Let S be the subtree with root a^{(i)} of T^{(i+1)} , then the tree T^{(i)} is defined as follows.

    ● Graft \sigma_{2i} so that a^{(i)} is its right-child;

    ● Transplant the trees S as the right-subtree of the vertex \sigma_{2i} ;

    ● Graft \sigma_{2i-1} as the left-child of \sigma_{2i} .

    We can illustrate this transformation by the following

    \begin{align*} v_1 < u_1 < v_2 < u_2 < \dots < v_{j-1} < u_{j-1} < v_j \end{align*}

    We note that the vertex \sigma_{2i-1} is the minimal leaf of T^{(i)} for 1 \le i \le m-1 . And then, we define \psi'(\sigma) = T^{(1)} .

    Example. We run the new algorithm to the examples \sigma = 748591623 in Example 3.2 and Fig. 2 in [11]. As n = 9 , we have five pairs

    \begin{align*} d_5(\sigma) & = (3),& d_4(\sigma) & = (6,2),& d_3(\sigma) & = (9,1),& d_2(\sigma) & = (8,5),& d_1(\sigma) & = (7,4). \end{align*}

    By Algorithm C, we get five trees sequentially

    \begin{align*} d_5(\sigma) & = (3),& d_4(\sigma) & = (6,2),& d_3(\sigma) & = (9,1),& d_2(\sigma) & = (8,5),& d_1(\sigma) & = (7,4). \end{align*}

    with

    \begin{align*} a^{(4)}& = 3, & a^{(3)}& = 2, & a^{(2)}& = 9, & a^{(1)}& = 5,\\ b^{(4)}& = 3, & b^{(3)}& = 2, & b^{(2)}&\text{ does not exist}, & b^{(1)}& = 5. \end{align*}

    Thus, the increasing 1-2 tree T^{(1)} obtained from \sigma under \psi' is the same with the tree obtained from \sigma under \psi in Example 3.2 and Fig. 2 in [11].

    Theorem 17. The two bijections \psi and \psi' from {{{\mathcal{DU} }}_{n,k}} to {{\mathcal{T}}_{n,k}} are equal.

    Proof. It is clear that (C2) is equivalent to (B1). Since the rule (B2a) just exchange two labels, but does not change the tree-structure, it is enough to show that (C1) is produced recursively from (B1) and (B2b).

    Assume that \sigma_{2i-1} > a^{(i)} > \sigma_{2i} for some 1 \le i \le m-1 . We obtain the right tree in the following by applying (B1) to the tree T^{(i+1)} .

    \begin{align*} a^{(4)}& = 3, & a^{(3)}& = 2, & a^{(2)}& = 9, & a^{(1)}& = 5,\\ b^{(4)}& = 3, & b^{(3)}& = 2, & b^{(2)}&\text{ does not exist}, & b^{(1)}& = 5. \end{align*}

    Due to \sigma_{2i-1} > v_1 ( = a^{(i)}) , it is not an increasing 1-2 tree and we apply (B2b) to the above tree as follows.

    \begin{align*} a^{(4)}& = 3, & a^{(3)}& = 2, & a^{(2)}& = 9, & a^{(1)}& = 5,\\ b^{(4)}& = 3, & b^{(3)}& = 2, & b^{(2)}&\text{ does not exist}, & b^{(1)}& = 5. \end{align*}

    Since \sigma_{2i-1} > v_j > \dots > v_2 , until we have an increasing 1-2 tree, repeat to apply (B2b) as follows.

    \begin{align*} a^{(4)}& = 3, & a^{(3)}& = 2, & a^{(2)}& = 9, & a^{(1)}& = 5,\\ b^{(4)}& = 3, & b^{(3)}& = 2, & b^{(2)}&\text{ does not exist}, & b^{(1)}& = 5. \end{align*}

    Since (C2a) is produced from the rule (B1) and (B2b), then Algorithm C follows Algorithm B.

    The first author's work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1C1B2008269).



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