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formerly Central European Journal of Mathematics

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Volume 14, Issue 1 (Jan 2016)

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New iterative codes for 𝓗-tensors and an application

Feng Wang
  • Corresponding author
  • College of Science, Guizhou Minzu University, Guiyang, Guizhou 550025, China
  • Email:
/ Deshu Sun
  • College of Science, Guizhou Minzu University, Guiyang, Guizhou 550025, China
  • Email:
Published Online: 2016-04-23 | DOI:Β https://doi.org/10.1515/math-2016-0022

Abstract

New iterative codes for identifying 𝓗 -tensor are obtained. As an application, some sufficient conditions of the positive definiteness for an even-order real symmetric tensor, i.e., an even-degree homogeneous polynomial form are given. Advantages of results obtained are illustrated by numerical examples.

Keywords: 𝓗-tensors; Symmetric tensors; Homogeneous polynomial; Positive definiteness; Irreducible

MSC 2010: 15A18; 15A21; 15A69

1 Introduction

Let C(R) be the complex(real) field and N = {1, 2, ... n}. We call π’œ = {ai1i2...im) an m-order n-dimensional complex(real) tensor, if ai1i2β‹―im∈C(R), where ij = 1, ..., n for j = 1, ..., m [1–3]. A tensor π’œ = (ai1i2...im) is called symmetric [4], if ai1i2β‹―im=aΟ€(i1i2β‹―im), β€‰β€‰βˆ€Ο€βˆˆΞ m, where Ξ m is the permutation group of m indices. Furthermore, an m-order n-dimensional tensor π“˜ = (Ξ΄i1i2...im) is called the unit tensor [5], if its entries Ξ΄i1i2β‹―im={1, if  i1=β‹―=im, 0, otherwise.

Consider the following positive definiteness identification problem [1].

Problem 1.1: For a real valued polynomial f, how to check whether f is positive definite, i.e., f(x)>0  for  any  x∈Rn,    xβ‰ 0, (1) or not?Problem 1.1 appears in numerous application domains [6–8]. In particular, the positive definiteness of multivariate polynomial f plays an important role in the stability study of nonlinear autonomous systems via Lyapunov’s direct method in automatic control [7, 9, 10], such as the multivariate network realizability theory [11], a test for Lyapunov stability in multivariate filters [12], a test of existence of periodic oscillations using Bendixon’s theorem [13], and the output feedback stabilization problems [14].

In 2005, Qi [1] defined the positive definiteness of a symmetric tensor π’œ, i.e., we call π’œ = (ai1i2...im) positive definite if the following mth-degree homogeneous polynomial f(x) is positive definite:

f(x)=Axm=βˆ‘i1, i2, …, im∈Nai1i2β‹―imxi1xi2...xim, (2)

where x = (x1, x2, .... xn) ∈ Rn Hence, we only research the positive definiteness of a symmetric tensor π’œ instead of f(x). Also in [1], Qi presented the concept of H-eigenvalues, and used it to verify the positive definiteness of an even-order symmetric tensor (see Proposition 1).

Proposition 1.2: ([1]). Let π’œ be an even-order real symmetric tensor, then π’œ is positive definite if and only if all of its H-eigenvalues are positive.From Proposition 1.2, we can verify the positive definiteness of an even-order symmetric tensor π’œ (the positive definiteness of the mth-degree homogeneous polynomial f(x)) by computing the H-eigenvalues of π’œ . But it is not easy to compute all these H-eigenvalues when m and n are large. Recently, by introducing the definition of 𝓗-tensors [15, 16], Li et al. [16] provided a practical sufficient condition for identifying the positive definiteness of an even-order symmetric tensor (see Proposition 1. 2).

Proposition 1.3: ([16]). Let π’œ = (ai1i2...im) be an even-order real symmetric tensor with ak ... k > 0 for all k ∈ N. If π’œ is an 𝓗-tensor, then π’œ is positive definite.Now, some definitions and notation are given, which will be used in the sequel.

Definition 1.4: ([1]). Let π’œ = (ai1i2...im) be an m-order n-dimensional complex tensor. If |aiiβ‹―i|β‰₯βˆ‘i2, …, im∈NΞ΄ii2….im=0|aii2β‹―im|, βˆ€i∈N.(3) then π’œ is called a diagonally dominant tensor; if all strict inequalities in (3) hold, then π’œ is called a strictly diagonally dominant tensor.

Definition 1.5: ([15]). Let π’œ = π’œ = (ai1...im) be an m-order n-dimensional complex tensor. π’œ is called an 𝓗-tensor if there is a positive vector x = (x1, x2, . . . xn)T ∈ Rn such that |aiiβ‹―i|ximβˆ’1>βˆ‘i2, …, im∈N, Ξ΄ii2β‹―im=0|aii2β‹―im| xi2β‹―xim,             i=1, 2, β‹―, n.

Definition 1.6: ([3]). Let π’œ = (ai1i2...im) be an m-order n-dimensional complex tensor, X = diag (x1, x2, ... xn). Denote B=(bi1β‹―im)=AXmβˆ’1,         bi1i2β‹―im=ai1i2β‹―imxi2xi3β‹―xim,         ij∈N,         j∈N, then 𝓑 is called the product of the tensor π’œ and the matrix X.

Definition 1.7: ([5]). An m-order n-dimensional complex tensor π’œ = (ai1...im) is called reducible, if there exists a nonempty proper index subset I βŠ‚ N such that ai1i2β‹―im=0, β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰βˆ€i1∈I, β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰βˆ€i2, …, imβˆ‰I.If π’œ is not reducible, then we call π’œ irreducible.Let S be a nonempty subset of N and let N\S be the complement of S in N . Given an m-order n-dimensional complex tensor π’œ = (ai1i2...im), we denote Ri(A)=βˆ‘i2, …, im∈NΞ΄ii2…im=0aii2β‹―im|=βˆ‘i2, …, im∈N|aii2β‹―im|βˆ’|aiiβ‹―i|, N1={i∈N:0<|aiiβ‹―i|=Ri(A)}, N2={i∈N:0<|aiiβ‹―i|<Ri(A)}, N3={i∈N:|aiiβ‹―i|>Ri(A)}, ΞΌi=Ri(A)βˆ’|aiiβ‹―i|Ri(A), βˆ€i∈N2, r0=1, r1=maxi∈N3Ri(A)|aiiβ‹―i|, Οƒk+1, i=βˆ‘i2β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2β‹―im∈N2mβˆ’1maxj∈{i2, β‹―, im}{ΞΌj}|aii2β‹―im|+rkβˆ‘i2β‹―im∈N3mβˆ’1Ξ΄ii2…im=0|aii2β‹―im|, βˆ€i∈N3, k∈Z+={0, 1, 2, β‹―}, rk+1=maxi∈N3Οƒk+1, i|aiiβ‹―i|, k=1, 2, 3, β‹―, Smβˆ’1={i2i3β‹―im:ij∈S, j=2, 3, β‹―, m}, Nmβˆ’1βˆ–Smβˆ’1=i2i3β‹―im:i2i3β‹―im∈Nmβˆ’1andi2i3β‹―imβˆ‰Smβˆ’1, N0mβˆ’1=Nmβˆ’1βˆ–(N2mβˆ’1βˆͺN3mβˆ’1).

It is well known that if N1 βˆͺ N2 = βˆ…, then π’œ is an 𝓗-tensor, and if π’œ is an 𝓗-tensor, then N3 βˆ… ; [16]. So we always assume that both N1 βˆͺ N2 and N3 are not empty. In addition, we also assume that π’œ satisfies: aii...i β‰  0,, Ri(π’œ) β‰  0, βˆ€i ∈ N.

This article is organized as follows: In Section 2, New iterative codes for identifying π’œ-tensors are obtained. As an application, some new sufficient conditions of the positive definiteness for an even-order real symmetric tensor are presented in Section 3. Numerical examples are given to verify the corresponding results.

2 Criteria for identifying π’œ-tensors

In this section, we give new iterative codes for identifying π’œ-tensors.

Lemma 2.1: ([15]). If π’œ is a strictly diagonally dominant tensor, then π’œ is an 𝓗-tensor.

Lemma 2.2: ([16]). Let π’œ = (ai1...im) be an m-order n-dimensional complex tensor. If π’œ is irreducible, |aiiβ‹―i|β‰₯Ri(A), β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰β€‰βˆ€i∈N, and strictly inequality holds for at least one i, then π’œ is an 𝓗-tensor.

Lemma 2.3: ([3]). Let π’œ = (ai1...im) be an m-order n-dimensional complex tensor. If there exists a positive diagonal matrix X such that π’œXm–1 is an 𝓗-tensor, then π’œ is an 𝓗-tensor.

Theorem 2.4: Let π’œ = (ai1...im) be an m-order n-dimensional complex tensor. If there exists k ∈ N such that |aiiβ‹―i|>βˆ‘i2β‹―im∈N0mβˆ’1Ξ΄ii2…im=0|aii2β‹―im|+βˆ‘i2β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|,βˆ€i∈N1,(4) and |aiiβ‹―i|ΞΌi>βˆ‘i2β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2β‹―im∈N2mβˆ’1Ξ΄ii2…im=0maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|,βˆ€i∈N2,(5) then π’œ is an 𝓗-tensor.

Proof: For all i ∈ N1, we denote Mi=1βˆ‘i2β‹―im∈N3mβˆ’1|aii2β‹―im||aiiβ‹―i|ΞΌiβˆ’βˆ‘i2β‹―im∈N0mβˆ’1Ξ΄ii2…im=0|aii2β‹―im|βˆ’βˆ‘i2β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|βˆ’βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|, and for all i ∈ N2, we denote Mi=1βˆ‘i2β‹―im∈N3mβˆ’1|aii2β‹―im||aiiβ‹―i|ΞΌiβˆ’βˆ‘i2β‹―im∈N0mβˆ’1|aii2β‹―im|βˆ’βˆ‘i2β‹―im∈N2mβˆ’1Ξ΄ii2…im=0maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|βˆ’βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|.If βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|=0, we denote Mi = +∞. From Inequalities (4) and (5), we obtain Mi < 0(i ∈ N1 ⋃ N2). Hence, there exists a positive number Ξ΅ such that 0<Ξ΅<minmini∈N1βˆͺN2Mi,1βˆ’maxj∈N3Οƒk+1,j|ajjβ‹―j|.(6)Let the matrix X = diag(x1, x2, . . . ; xn), where xi=1, i∈N1, ΞΌi1mβˆ’1, i∈N2, Ξ΅+Οƒk+1, i|aiiβ‹―i|1mβˆ’1, i∈N3.By Inequality (6), we have (Ξ΅+Οƒk+1, i|aiiβ‹―i|)1mβˆ’1<1(βˆ€i∈N3). As Ξ΅ β‰  +∞, xi β‰  +∞, which implies that X is a diagonal matrix with positive entries. Let 𝓑 = (bi1i2...im) = π’œXm βˆ’ 1. Next, we will prove that 𝓑 is strictly diagonally dominant.First, we consider i ∈ N1. If βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|=0, then by Inequality (4), we have Ri(B)=βˆ‘i2i3β‹―im∈N0mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|xi2β‹―xim+βˆ‘i2i3β‹―im∈N2mβˆ’1|aii2β‹―im|(ΞΌi2)1mβˆ’1β‹―(ΞΌim)1mβˆ’1β‰€βˆ‘i2i3β‹―im∈N0mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|<|aiiβ‹―i|=|biiβ‹―i|.(7)If βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|β‰ 0, then by Inequalities (4) and (6), we get Ri(B)=βˆ‘i2i3β‹―im∈N0mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|xi2β‹―xim+βˆ‘i2i3β‹―im∈N2mβˆ’1|aii2β‹―im|(ΞΌi2)1mβˆ’1β‹―(ΞΌim)1mβˆ’1+βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|Ξ΅+Οƒk+1,i2|ai2i2β‹―i2|1mβˆ’1β‹―Ξ΅+Οƒk+1,im|aimimβ‹―im|1mβˆ’1β‰€βˆ‘i2i3β‹―im∈N0mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|Ξ΅+maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j|=βˆ‘i2i3β‹―im∈N0mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|+Ξ΅βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|<|aiiβ‹―i|=|biiβ‹―i|.(8)Next, we consider i ∈ N2. If βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|=0, then by Inequality (5), we have Ri(B)=βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|xi2β‹―xim+βˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|(ΞΌi2)1mβˆ’1β‹―(ΞΌim)1mβˆ’1β‰€βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|<|aiiβ‹―i|ΞΌi=|biiβ‹―i|.(9) If βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|β‰ 0, then by Inequalities (5) and (6), we get Ri(B)=βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|xi2β‹―xim+βˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|(ΞΌi2)1mβˆ’1β‹―(ΞΌim)1mβˆ’1+βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|Ξ΅+Οƒk+1,i2|ai2i2β‹―i2|1mβˆ’1β‹―Ξ΅+Οƒk+1,im|aimimβ‹―im|1mβˆ’1β‰€βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|Ξ΅+maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j|=βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|+ΞΌβˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|+Ξ΅βˆ‘i2i3β‹―im∈N3mβˆ’1|aii2β‹―im|<|aiiβ‹―i|ΞΌi=|biiβ‹―i|.(10) Finally, we consider i ∈ N3. Since |aii... i| > Ri(π’œ), we have |aiiβ‹―i|βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|>0.(11) And by Οƒk+1, j|ajjβ‹―j|≀rk+1≀rk, βˆ€k∈N, j∈N3, we obtain βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|βˆ’Οƒk+1,i=βˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|βˆ’rkβˆ‘i2i3β‹―im∈N2mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|≀0.(12) By Inequalities (11), (12) and Ξ΅ > 0, we get Ξ΅>1|aiiβ‹―i|βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|{βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|βˆ’Οƒk+1,i}.(13) From Inequality (13), for any i ∈ N3, we have |biiβ‹―i|βˆ’Ri(B)=|aiiβ‹―i|Ξ΅+Οƒk+1,i|aiiβ‹―i|βˆ’βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|xi2β‹―ximβˆ’βˆ‘i2i3β‹―im∈N2mβˆ’1|aii2β‹―im|(ΞΌi2)1mβˆ’1β‹―(ΞΌim)1mβˆ’1βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|Ξ΅+Οƒk+1,i2|ai2i2β‹―i2|1mβˆ’1β‹―Ξ΅+Οƒk+1,im|aimimβ‹―im|1mβˆ’1β‰₯|aiiβ‹―i|Ξ΅+Οƒk+1,i|aiiβ‹―iβˆ’βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|βˆ’βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|Ξ΅+maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j|=Ξ΅(|aiiβ‹―i|βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0|aii2β‹―im|)+Οƒk+1,iβˆ’βˆ‘i2i3β‹―im∈N0mβˆ’1|aii2β‹―im|βˆ’βˆ‘i2i3β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|βˆ’βˆ‘i2i3β‹―im∈N3mβˆ’1Ξ΄ii2β‹―im=0maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|>0.(14) Therefore, from Inequalities (7-10) and (14), we obtain |bii... i| > Ri(𝓑) for all i ∈ N, and by Lemma 2.1, 𝓑 is an 𝓗-tensor. Furthermore, by Lemma 2.3, π’œ is an 𝓗-tensor. The proof is completed.

Theorem 2.5: Let π’œ = (ai1 ...im) be an m-order n-dimensional complex tensor. If A is irreducible, and there exists k ∈ Z C such that |aiiβ‹―i|β‰₯βˆ‘i2β‹―im∈N0mβˆ’1Ξ΄ii2…im=0|aii2β‹―im|+βˆ‘i2β‹―im∈N2mβˆ’1maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|,βˆ€i∈N1,(15) and |aiiβ‹―i|ΞΌiβ‰₯βˆ‘i2β‹―im∈N0mβˆ’1|aii2β‹―im|+βˆ‘i2β‹―im∈N2mβˆ’1Ξ΄ii2…im=0maxj∈{i2,β‹―,im}ΞΌj|aii2β‹―im|+βˆ‘i2β‹―im∈N3mβˆ’1maxj∈{i2,i3,β‹―,im}Οƒk+1,j|ajjβ‹―j||aii2β‹―im|,βˆ€i∈N2,(16) in addition, a strict inequality holds for at least one i ∈ N1 ⋃ N2, then 𝓐 is an 𝓗-tensor.

Proof: Let the matrix X = d iag (x1, x2, . . . ; xn), where xi={1, i∈N1, (ΞΌi)1mβˆ’1, i∈N2, (Οƒk+1, i|aiiβ‹―i|)1mβˆ’1, i∈N3.By the irreducibility of π’œ, we have xi β‰  +∞, then X is a diagonal matrix with positive diagonal entries. Let 𝓑 = (bi1...im) = π’œXm βˆ’1.Adopting the same procedure as in the proof of Theorem 2.4, we can obtain that |bii...i| β‰₯ Ri(𝓑)(βˆ€i ∈ N), and there exists at least an i ∈ N1 βˆͺ N2 such that |bii...i| β‰₯ Ri (𝓑).On the other hand, since π’œ is irreducible, then 𝓑 is also. Then by Lemma 2.2, we have that 𝓑 is an 𝓗-tensor. By Lemma 2.3, π’œ is an 𝓗-tensor. The proof is completed. β–‘

Example 2.6: Consider an 3-order 3-dimensional tensor π’œ = (aijk) defined as follows: A=[A(1, :, :), A(2, :, :), A(3, :, :)], ], [A(1, :, :)=(121011001110), A(2, :, :)=(1100180103), A(3, :, :)=(2000300015).Obviously, |a111|=12, R1(A)=24, |a222|=18, R2(A)=6, |a333|=15, R3(A)=5, so N1 = βˆ…, N2 = {1}, N3 = {2; 3}. By calculation, we have ΞΌ1=12, Οƒ3, 2|a222|=37216, Οƒ3, 3|a333|=19180.Since βˆ‘jk∈N02|a1jk|+βˆ‘jk∈N22Ξ΄1jk=0maxl∈{j,k}ΞΌl|a1jk|+βˆ‘jk∈N32maxl∈{j,k}Οƒk+1,l|allβ‹―l||a1jk|=(1+0+1+1)+37216+10Γ—37216+10Γ—19180=1283216<6=|a11β‹―1|ΞΌ1,we know that A satisfies the conditions (k = 2) of Theorem 2.4, then π’œ is an 𝓗-tensor.

3 An application

In this section, based on the criteria of 𝓗-tensors in section 2, we present new conditions for identifying the positive definiteness of an even-order real symmetric tensor.

From Proposition 2, Theorem 2.4 and Theorem 2.5, we obtain easily the following result.

Theorem 3.1: Let π’œ = ai1 ...im) be an m-order n-dimensional even-order real symmetric tensor with akk...k > 0 for all k ∈ N. If π’œ satisfies one of the following conditions, then π’œ is positive definite,

Example 3.2: Let f(x)=Ax4=2x14+x24+30x34+30x44βˆ’8x1x33+8x2x43βˆ’12x33x4be a 4th-degree homogeneous polynomial. We can get an 4-order 4-dimensional real symmetric tensor π’œ = (ai1i2i3i4), where a1111=2, a2222=1, a3333=30, a4444=30, a1333=a3133=a3313=a3331=βˆ’2, a2444=a4244=a4424=a4442=2, a3334=a3343=a3433=a4333=βˆ’3, and other ai1i2i3i4 = 0. It can be verified that A satisfies all the conditions (k = 2) of Theorem 2.4. Therefore, from Theorem 3.1, we have that π’œ is positive definite, that is, f(x) is positive definite.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (11501141, 11361074), the Foundation of Science and Technology Department of Guizhou Province ([2015]2073, [2015]7206), the Natural Science Programs of Education Department of Guizhou Province ([2015]420), and the Research Foundation of Guizhou Minzu University (15XRY004).

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About the article

Received: 2015-11-21

Accepted: 2016-03-14

Published Online: 2016-04-23

Published in Print: 2016-01-01



Citation Information: Open Mathematics, ISSN (Online) 2391-5455, DOI: https://doi.org/10.1515/math-2016-0022. Export Citation

Β© 2016 Wang and Sun, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. (CC BY-NC-ND 3.0)

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