ALAMT  Vol.7 No.3 , September 2017
A Note on the Inclusion Sets for Tensors
ABSTRACT
In this paper, we give a note on the eigenvalue localization sets for tensors. We show that these sets are tighter than those provided by Li et al. (2014) [1].

1. Introduction

Eigenvalue problems of higher order tensors have become an important topic of study in a new applied mathematics branch, numerical multilinear algebra, and they have a wide range of practical applications [2] - [9] .

First, we recall some definitions on tensors. Let be the real field. An m-th order n dimensional square tensor A consists of nm entries in , which is defined as follows:

A = ( a i 1 i 2 i m ) , a i 1 i 2 i m , 1 i 1 , i 2 , i m n .

To an n-vector x, real or complex, we define the n-vector:

A x m 1 = ( i 2 , , i m = 1 n a i i 2 i m x i 2 x i m ) 1 i n .

and

x [ m 1 ] = ( x i m 1 ) 1 i n .

If A x m 1 = λ x [ m 1 ] , x and λ are all real, then λ is called an H-eigenvalue of A and x an H-eigenvector of A associated with λ [10] [11] .

Qi [10] generalized Geršgorin eigenvalue inclusion theorem from matrices to real supersymmetric tensors, which can be easily extended to generic tensors; see [1] .

Theorem 1. Let A = ( a i 1 i 2 i m ) be a complex tensor of order m dimension n . Then

σ ( A ) Γ ( A ) = i N Γ i ( A )

where τ ( A ) is the set of all the eigenvalues of A and

Γ i ( A ) = { z : | z a i i | r i ( A ) } ,

where

δ i 1 i m = { 1, if i 1 = = i m 0, otherwise ,

and

r i ( A ) = δ i i 2 i m = 0 | a i i 2 i m | .

Recently, Li et al. [1] obtained the following result, which is also used to identify the positive definiteness of an even-order real supersymmetric tensor.

Theorem 2. Let A = ( a i 1 i 2 i m ) be a complex tensor of order m dimension n . Then

σ ( A ) K ( A ) = i , j N , i j K i , j ( A )

where σ ( A ) is the set of all the eigenvalues of A and

K i , j ( A ) = { z : ( | z a i i | r i j ( A ) ) | z a j j | | a i j j | r j ( A ) } ,

where

r i j ( A ) = δ i i 2 i m = 0 , δ j i 2 i m = 0 | a i i 2 i m | = r i ( A ) | a i j j | .

In this paper, we give some new eigenvalue localization sets for tensors, which are tighter than those provided by Li et al. [1] .

2. New Eigenvalue Inclusion Sets

Theorem 3. Let A = ( a i 1 i 2 i m ) be a complex tensor of order m dimension n . Then

σ ( A ) Δ ( A ) = i N j N , j i Δ i , j ( A )

where σ ( A ) is the set of all the eigenvalues of A and

Δ i , j ( A ) = { z : | z a i i | ( | z a j j | r j i ( A ) ) | a j i i | r i ( A ) } ,

where

r j i ( A ) = δ j i 2 i m = 0 , δ i i 2 i m = 0 | a j i 2 i m | = r j ( A ) | a j i i | .

Proof. Let x = ( x 1 , , x n ) T be an eigenvector of A corresponding to λ ( A ) , that is,

A x m 1 = λ x [ m 1 ] . (1)

Let

| x p | = m a x { | x i | , i N } .

Obviously, | x p | > 0 . For any q p , from equality (1), we have

| λ a p p | | x p | m 1 δ p i 2 i m = 0 | a p i 2 i m | | x i 2 | | x i m | δ q i 2 i m = 0 , δ p i 2 i m = 0 | a p i 2 i m | | x i 2 | | x i m | + | a p q q | | x q | m 1 δ q i 2 i m = 0 , δ p i 2 i m = 0 | a p i 2 i m | | x p | m 1 + | a p q q | | x q | m 1 r p q ( A ) | x p | m 1 + | a p q q | | x q | m 1 . (2)

That is,

( | λ a p p | r p q ( A ) ) | x p | m 1 | a p q q | | x q | m 1 . (3)

If | x q | = 0 for all q p , then | λ a p p | r p q ( A ) 0 , and λ Δ ( A ) . If | x q | > 0 , from equality (1), we have

| λ a q q | | x q | m 1 r q ( A ) | x p | m 1 . (4)

Multiplying inequalities (3) with (4), we have

| λ a q q | ( | λ a p p | r p q ( A ) ) r q ( A ) | a p q q | , (5)

which implies that λ Δ p , q ( A ) . From the arbitrariness of q, we have λ Δ ( A ) . ,

Remark 1. Obviously, we can get K ( A ) Δ ( A ) . That is to say, our new eigenvalue inclusion sets are always tighter than the inclusion sets in Theorem 2.

Remark 2. If the tensor A is nonnegative, from (5), we can get

( λ a q q ) ( λ a p p r p q ( A ) ) r q ( A ) a p q q .

Then, we can get,

λ 1 2 { a p p + a q q + r p q ( A ) + Θ p , q 1 2 ( A ) }

where

Θ p , q ( A ) = ( a p p a q q + r p q ( A ) ) 2 + 4 a p q q r q ( A ) .

From the arbitrariness of q, we have

λ m a x i N m i n j N , j i 1 2 { a j j + a i i + r j i ( A ) + Θ j , i 1 2 ( A ) } .

That is to say, from Theorem 3, we can get another proof of the result in Theorem 13 in [12] .

Funds

Jun He is supported by Science and technology Foundation of Guizhou province (Qian ke he Ji Chu [2016]1161); Guizhou province natural science foundation in China (Qian Jiao He KY [2016]255); The doctoral scientific research foundation of Zunyi Normal College (BS [2015]09); High-level innovative talents of Guizhou Province (Zun Ke He Ren Cai [2017]8). Yan-Min Liu is supported by National Natural Science Foundations of China (71461027); Science and technology talent training object of Guizhou province outstanding youth (Qian ke he ren zi [2015]06); Guizhou province natural science foundation in China (Qian Jiao He KY [2014]295); 2013, 2014 and 2015 Zunyi 15,851 talents elite project funding; Zhunyi innovative talent team(Zunyi KH (2015)38). Tian is supported by Guizhou province natural science foundation in China (Qian Jiao He KY [2015]451); Scienceand technology Foundation of Guizhou province (Qian ke he J zi [2015]2147). Xiang-Hu Liu is supported by Guizhou Province Department of Education Fund KY [2015]391, [2016]046; Guizhou Province Department of Education teaching reform project [2015]337; Guizhou Province Science and technology fund (qian ke he ji chu) [2016]1160.

Cite this paper
He, J. , Liu, Y. , Tian, J. and Liu, X. (2017) A Note on the Inclusion Sets for Tensors. Advances in Linear Algebra & Matrix Theory, 7, 67-71. doi: 10.4236/alamt.2017.73006.
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