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An S-type singular value inclusion set for rectangular tensors
Journal of Inequalities and Applications volume 2017, Article number: 141 (2017)
Abstract
An S-type singular value inclusion set for rectangular tensors is given. Based on the set, new upper and lower bounds for the largest singular value of nonnegative rectangular tensors are obtained and proved to be sharper than some existing results. Numerical examples are given to verify the theoretical results.
1 Introduction
Let \(\mathbb{R} (\mathbb{C})\) be the real (complex) field, \(p,q,m,n\) be positive integers, \(l=p+q\), \(m,n\geq2\) and \(N=\{1,2,\ldots,n\}\). We call \(\mathcal{A}=(a_{i_{1}\cdots i_{p}j_{1}\cdots j_{q}})\) a real \((p,q)\)th order \(m\times n\) dimensional rectangular tensor, or simply a real rectangular tensor, denoted by \(\mathcal{A}\in\mathbb{R}^{[p,q;m,n]}\), if
When \(p=q=1, \mathcal{A}\) is simply a real \(m\times n\) rectangular matrix. This justifies the word ‘rectangular’. We call \(\mathcal{A}\) nonnegative, denoted by \(\mathcal{A}\in\mathbb {R}^{[p,q;m,n]}_{+}\), if each of its entries \(a_{i_{1}\cdots i_{p}j_{1}\cdots j_{q}}\geq0\).
For any vectors \(x=(x_{1},x_{2},\ldots, x_{m})^{\mathrm{T}}\), \(y=(y_{1},y_{2},\ldots , y_{n})^{\mathrm{T}}\) and any real number α, denote \(x^{[\alpha ]}=(x_{1}^{\alpha},x_{2}^{\alpha},\ldots,x_{m}^{\alpha})^{\mathrm{T}}\) and \(y^{[\alpha]}=(y_{1}^{\alpha}, y_{2}^{\alpha},\ldots, y_{n}^{\alpha})^{\mathrm{T}}\). Let \(\mathcal{A}x^{p-1}y^{q}\) be a vector in \(\mathbb{R}^{m}\) such that
where \(i=1,\ldots,m\). Similarly, let \(\mathcal{A}x^{p}y^{q-1}\) be a vector in \(\mathbb{R}^{n}\) such that
where \(j=1,\ldots,n\). If there are a number \(\lambda\in\mathbb{C}\), vectors \(x\in\mathbb{C}^{m}\backslash\{ 0\}\), and \(y\in\mathbb{C}^{n}\backslash\{0\}\) such that
then λ is called the singular value of \(\mathcal{A}\), and \((x,y)\) is a pair of left and right eigenvectors of \(\mathcal{A}\), associated with λ, respectively. If \(\lambda\in\mathbb{R}, x\in\mathbb{R}^{m}\), and \(y\in\mathbb{R}^{n}\), then we say that λ is an H-singular value of \(\mathcal{A}\), and \((x,y)\) is a pair of left and right H-eigenvectors associated with λ, respectively. If a singular value is not an H-singular value, we call it an N-singular value of \(\mathcal{A}\) [1]. We call
the largest singular value [2].
Note here that the definition of singular values for tensors was first proposed by Lim in [3]. When l is even, the definition in [1] is the same as in [3]. When l is odd, the definition in [1] is slightly different from that in [3], but parallel to the definition of eigenvalues of square matrices [4]; see [1] for details.
When \(m=n\), such real rectangular tensors have a sound application background. For example, the elasticity tensor is a tensor with \(p=q=2\) and \(m=n=2\) or 3; for details, see [1]. Due to the fact that singular values of rectangular tensors have a wide range of practical applications in the strong ellipticity condition problem in solid mechanics [5, 6] and the entanglement problem in quantum physics [7, 8], very recently, it has attracted attention of researchers [9–17]. Chang et al. [1] studied some properties of singular values of rectangular tensors, which include the Perron-Frobenius theorem of nonnegative irreducible tensors. Yang et al. [2] extended the Perron-Frobenius theorem of nonnegative irreducible tensors to nonnegative tensors, and gave the upper and lower bounds of the largest singular value of nonnegative rectangular tensors.
Our goal in this paper is to propose a singular value inclusion set for rectangular tensors and use the set to obtain new upper and lower bounds for the largest singular value of nonnegative rectangular tensors.
2 Main results
In this section, we begin with some notation. Let \(\mathcal{A}\in \mathbb{R}^{[p,q;n,n]}\). For \(\forall i,j\in N, i\neq j\), denote
where
Theorem 1
Let \(\mathcal{A}\in\mathbb{R}^{[p,q;n,n]}\), S be a nonempty proper subset of N, SÌ„ be the complement of S in N. Then
where
Proof
For any \(\lambda\in\sigma(\mathcal{A})\), let \(x=(x_{1},x_{2},\ldots ,x_{n})^{\mathrm{T}}\in\mathbb{C}^{n}\backslash\{0\}\) and \(y=(y_{1},y_{2},\ldots ,y_{n})^{\mathrm{T}}\in\mathbb{C}^{n}\backslash\{0\}\) be the associated left and right eigenvectors, that is,
Let
Then, at least one of \(\vert x_{s} \vert \) and \(\vert x_{t} \vert \) is nonzero, and at least one of \(\vert y_{g} \vert \) and \(\vert y_{h} \vert \) is nonzero. We divide the proof into four parts.
Case I: Suppose that \(w_{S}= \vert x_{s} \vert , w_{\bar{S}}= \vert x_{t} \vert \), then \(\vert x_{s} \vert \geq \vert y_{s} \vert , \vert x_{t} \vert \geq \vert y_{t} \vert \).
(i) If \(\vert x_{s} \vert \geq \vert x_{t} \vert \), then \(\vert x_{s} \vert =\max_{i\in N} \{w_{i}\}\). The sth equality in (1) is
Taking modulus in the above equation and using the triangle inequality give
i.e.,
If \(\vert x_{t} \vert =0\), then \(\vert \lambda \vert -r_{s}^{t}(\mathcal{A})\leq0\) as \(\vert x_{s} \vert >0\), and it is obvious that
which implies that \(\lambda\in\hat{\Upsilon}_{s,t}(\mathcal{A})\). Otherwise, \(\vert x_{t} \vert >0\). Moreover, from the tth equality in (1), we can get
Multiplying (3) by (4) and noting that \(\vert x_{s} \vert ^{l-1} \vert x_{t} \vert ^{l-1}>0\), we have
which also implies that \(\lambda\in\hat{\Upsilon}_{s,t}(\mathcal{A})\subseteq\bigcup_{i\in S,j\in\bar{S}}\hat{\Upsilon}_{i,j}(\mathcal{A})\).
(ii) If \(\vert x_{t} \vert \geq \vert x_{s} \vert \), then \(\vert x_{t} \vert =\max_{i\in N} \{w_{i}\}\). Similarly, we can get
and \(\lambda\in\hat{\Upsilon}_{t,s}(\mathcal{A})\subseteq\bigcup_{i\in \bar{S},j\in S}\hat{\Upsilon}_{i,j}(\mathcal{A})\).
Case II: Suppose that \(w_{S}= \vert y_{g} \vert , w_{\bar{S}}= \vert y_{h} \vert \), then \(\vert y_{g} \vert \geq \vert x_{g} \vert , \vert y_{h} \vert \geq \vert x_{h} \vert \).
(i) If \(\vert y_{g} \vert \geq \vert y_{h} \vert \), then \(\vert y_{g} \vert =\max_{i\in N} \{w_{i}\}\). The gth equality in (2) is
Taking modulus in the above equation and using the triangle inequality give
i.e.,
If \(\vert y_{h} \vert =0\), then \(\vert \lambda \vert -c_{g}^{h}(\mathcal{A})\leq0\) as \(\vert y_{g} \vert >0\), and furthermore
which implies that \(\lambda\in\tilde{\Upsilon}_{g,h}(\mathcal{A})\). Otherwise, \(\vert y_{h} \vert >0\). Moreover, from the hth equality in (2), we can get
Multiplying (5) by (6) and noting that \(\vert y_{g} \vert ^{l-1} \vert y_{h} \vert ^{l-1}>0\), we have
which also implies that \(\lambda\in\tilde{\Upsilon}_{g,h}(\mathcal {A})\subseteq\bigcup_{i\in S,j\in\bar{S}}\tilde{\Upsilon }_{i,j}(\mathcal{A})\).
(ii) If \(\vert y_{h} \vert \geq \vert y_{g} \vert \), then \(\vert y_{h} \vert =\max_{i\in N} \{w_{i}\}\). Similarly, we can get
and \(\lambda\in\tilde{\Upsilon}_{h,g}(\mathcal{A})\subseteq\bigcup_{i\in \bar{S},j\in S}\tilde{\Upsilon}_{i,j}(\mathcal{A})\).
Case III: Suppose that \(w_{S}= \vert x_{s} \vert , w_{\bar{S}}= \vert y_{h} \vert \), then \(\vert x_{s} \vert \geq \vert y_{s} \vert , \vert y_{h} \vert \geq \vert x_{h} \vert \). If \(\vert x_{s} \vert \geq \vert y_{h} \vert \), then \(\vert x_{s} \vert =\max_{i\in N} \{w_{i}\}\). Similar to the proof of (3) and (6), we have
and
Furthermore, we have
which implies that \(\lambda\in\hat{\Upsilon}_{s,h}(\mathcal{A})\subseteq\bigcup_{i\in S,j\in\bar{S}}\hat{\Upsilon}_{i,j}(\mathcal{A})\). And if \(\vert y_{h} \vert \geq \vert x_{s} \vert \), then \(\vert y_{h} \vert =\max_{i\in N} \{w_{i}\}\). Similarly, we can get
which implies that \(\lambda\in\tilde{\Upsilon}_{h,s}(\mathcal{A})\subseteq\bigcup_{i\in \bar{S},j\in S}\tilde{\Upsilon}_{i,j}(\mathcal{A})\).
Case IV: Suppose that \(w_{S}= \vert y_{g} \vert , w_{\bar{S}}= \vert x_{t} \vert \), then \(\vert y_{g} \vert \geq \vert x_{g} \vert , \vert x_{t} \vert \geq \vert y_{t} \vert \). If \(\vert y_{g} \vert \geq \vert x_{t} \vert \), then \(\vert y_{g} \vert =\max_{i\in N} \{w_{i}\}\). Similar to the proof of (5) and (4), we have
and
Furthermore, we have
which implies that \(\lambda\in\tilde{\Upsilon}_{g,t}(\mathcal {A})\subseteq\bigcup_{i\in\bar{S},j\in S}\tilde{\Upsilon }_{i,j}(\mathcal{A})\). And if \(\vert x_{t} \vert \geq \vert y_{g} \vert \), then \(\vert x_{t} \vert =\max_{i\in N} \{w_{i}\}\). Similarly, we can get
which implies that \(\lambda\in\hat{\Upsilon}_{t,g}(\mathcal {A})\subseteq\bigcup_{i\in\bar{S},j\in S}\hat{\Upsilon}_{i,j}(\mathcal{A})\). The proof is completed. □
Based on Theorem 1, bounds for the largest singular value of nonnegative rectangular tensors are given.
Theorem 2
Let \(\mathcal{A}=(a_{i_{1}\cdots i_{m}})\in\mathbb {R}^{[p,q;n,n]}_{+}\), S be a nonempty proper subset of N, SÌ„ be the complement of S in N. Then
where
and
Proof
First, we prove that the second inequality in (7) holds. By Theorem 2 in [2], we know that \(\lambda_{0}\) is a singular value of \(\mathcal{A}\). Hence, by Theorem 1, \(\lambda_{0}\in\Upsilon^{S}(\mathcal{A})\), that is,
If \(\lambda_{0}\in\bigcup_{i\in S,j\in\bar{S}} (\hat{\Upsilon }_{i,j}(\mathcal{A})\cup\tilde{\Upsilon}_{i,j}(\mathcal{A}) )\), then there are \(i\in S,j\in\bar{S}\) such that \(\lambda_{0}\in\hat{\Upsilon}_{i,j}(\mathcal{A})\) or \(\lambda_{0}\in\tilde {\Upsilon}_{i,j}(\mathcal{A})\). When \(\lambda_{0}\in\hat{\Upsilon}_{i,j}(\mathcal{A})\), i.e., \((\lambda_{0}-r_{i}^{j}(\mathcal{A}))\lambda_{0}\leq a_{ij\cdots jj\cdots j}\max \{R_{j}(\mathcal{A}),C_{j}(\mathcal{A})\}\), then solving \(\lambda_{0}\) gives
When \(\lambda_{0}\in\tilde{\Upsilon}_{i,j}(\mathcal{A})\), i.e., \((\lambda_{0}-c_{i}^{j}(\mathcal{A}))\lambda_{0}\leq a_{j\cdots jij\cdots j}\max \{R_{j}(\mathcal{A}),C_{j}(\mathcal{A})\}\), then solving \(\lambda_{0}\) gives
And if \(\lambda_{0}\in\bigcup_{i\in\bar{S},j\in S} (\hat{\Upsilon }_{i,j}(\mathcal{A})\cup\tilde{\Upsilon}_{i,j}(\mathcal{A}) )\), similarly, we can obtain that \(\lambda_{0}\leq\hat{U}^{\bar{S}}(\mathcal{A})\) and \(\lambda_{0}\leq\tilde {U}^{\bar{S}}(\mathcal{A})\).
Second, we prove that the first inequality in (7) holds. Assume that \(\mathcal{A}\) is an irreducible nonnegative rectangular tensor, by Theorem 6 of [1], then \(\lambda_{0}>0\) with two positive left and right associated eigenvectors \(x=(x_{1},x_{2},\ldots,x_{n})^{\mathrm{T}}\) and \(y=(y_{1},y_{2},\ldots,y_{n})^{\mathrm{T}}\). Let
We divide the proof into four parts.
Case I: Suppose that \(w_{S}=x_{s}, w_{\bar{S}}=x_{t}\), then \(y_{s}\geq x_{s}, y_{t}\geq x_{t}\).
(i) If \(x_{t}\geq x_{s}\), then \({x_{s}}=\min_{i\in N} \{w_{i}\}\). From the sth equality in (1), we have
i.e.,
Moreover, from the tth equality in (1), we can get
Multiplying (8) by (9) and noting that \(x_{s}^{l-1}x_{t}^{l-1}>0\), we have
Then solving for \(\lambda_{0}\) gives
(ii) If \(x_{s}\geq x_{t}\), then \(x_{t}=\min_{i\in N} \{w_{i}\}\). Similarly, we can get
Case II: Suppose that \(w_{S}=y_{g}, w_{\bar{S}}=y_{h}\), then \(x_{g}\geq y_{g}, x_{h}\geq y_{h}\).
(i) If \(y_{h}\geq y_{g}\), then \(y_{g}=\min_{i\in N} \{w_{i}\}\). From the gth equality in (2), we have
i.e.,
Moreover, from the hth equality in (2), we can get
Multiplying (10) by (11) and noting that \(y_{g}^{l-1}y_{h}^{l-1}>0\), we have
which gives
(ii) If \(y_{g}\geq y_{h}\), then \(y_{h}=\min_{i\in N} \{w_{i}\}\). Similarly, we can get
Case III: Suppose that \(w_{S}=x_{s}, w_{\bar{S}}=y_{h}\), then \(y_{s}\geq x_{s}, x_{h}\geq y_{h}\). If \(y_{h}\geq x_{s}\), then \(x_{s}=\min_{i\in N} \{w_{i}\}\). Similar to the proof of (8) and (11), we have
and
Furthermore, we have
and
And if \(x_{s}\geq y_{h}\), then \(y_{h}=\min_{i\in N} \{w_{i}\}\). Similarly, we have
Case IV: Suppose that \(w_{S}=y_{g}, w_{\bar{S}}=x_{t}\), then \(x_{g}\geq y_{g}, y_{t}\geq x_{t}\). If \(x_{t}\geq y_{g}\), then \(y_{g}=\min_{i\in N} \{w_{i}\}\). Similar to the proof of (10) and (9), we have
and
Furthermore, we have
and
And if \(y_{g}\geq x_{t}\), then \(x_{t}=\min_{i\in N} \{w_{i}\}\). Similarly, we have
Assume that \(\mathcal{A}\) is a nonnegative rectangular tensor, then by Lemma 3 of [2] and similar to the proof of Theorem 2 of [2], we can prove that the first inequality in (7) holds. The conclusion follows from what we have proved. □
Next, a comparison theorem for these bounds in Theorem 2 and Theorem 4 of [2] is given.
Theorem 3
Let \(\mathcal{A}=(a_{i_{1}\cdots i_{m}})\in\mathbb {R}^{[p,q;n,n]}_{+}\), S be a nonempty proper subset of N. Then the bounds in Theorem 2 are better than those in Theorem 4 of [2], that is,
Proof
Here, only \(L^{S}(\mathcal{A})=\min\{\hat{L}^{S}(\mathcal{A}),\hat{L}^{\bar {S}}(\mathcal{A}),\tilde{L}^{S}(\mathcal{A}),\tilde{L}^{\bar{S}}(\mathcal {A})\}\geq\min_{1\leq i,j\leq n} \{R_{i}(\mathcal{A}),C_{j}(\mathcal {A}) \}\) is proved. Similarly, we can also prove that \(U^{S}(\mathcal{A})\leq\max_{1\leq i, j\leq n} \{R_{i}(\mathcal {A}),C_{j}(\mathcal{A}) \}\). Without loss of generality, assume that \(L^{S}(\mathcal{A})=\hat{L}^{S}(\mathcal{A})\), that is, there are two indexes \(i\in S,j\in\bar{S}\) such that
(we can prove it similarly if \(L^{S}(\mathcal{A})=\hat{L}^{\bar {S}}(\mathcal{A}),\tilde{L}^{S}(\mathcal{A}),\tilde{L}^{\bar{S}}(\mathcal {A})\), respectively). Now, we divide the proof into two cases as follows.
Case I: Assume that
(i) If \(R_{i}(\mathcal{A})\geq R_{j}(\mathcal{A})\), then \(a_{ij\cdots jj\cdots j}\geq R_{j}(\mathcal{A})-r_{i}^{j}(\mathcal{A})\). When \(R_{j}(\mathcal{A})-r_{i}^{j}(\mathcal{A})>0\), we have
And when \(R_{j}(\mathcal{A})-r_{i}^{j}(\mathcal{A})\leq0\), i.e., \(r_{i}^{j}(\mathcal{A})\geq R_{j}(\mathcal{A})\), we have
(ii) If \(R_{i}(\mathcal{A})< R_{j}(\mathcal{A})\), then
Case II: Assume that
Similar to the proof of Case I, we have \(L^{S}(\mathcal{A})\geq\min_{1\leq i, j\leq n} \{R_{i}(\mathcal {A}),C_{j}(\mathcal{A}) \}\). The conclusion follows from what we have proved. □
3 Numerical examples
In the following, two numerical examples are given to verify the theoretical results.
Example 1
Let \(\mathcal{A}\in\mathbb{R}^{[2,2;3,3]}_{+}\) with entries defined as follows:
By computation, we get that all different singular values of \(\mathcal {A}\) are \(-4.9395, -0.5833\), \(-0.4341, -0.1977, 0, 0.0094, 0.0907, 1.0825, 1.2405, 1.5334, 1.8418, 2.3125, 5.8540, 6.1494\), \(6.6525, 8.0225\) and 31.1680.
(i) An S-type singular value inclusion set.
Let \(S=\{1\}\). Obviously, \(\bar{S}=\{2,3\}\). By Theorem 1, the S-type singular inclusion set is
The singular value inclusion set \(\Upsilon^{S}(\mathcal{A})\) and the exact singular values are drawn in Figure 1, where \(\Upsilon^{S}(\mathcal{A})\) is represented by black solid boundary and the exact singular values are plotted by red ‘+’. It is easy to see that \(\Upsilon^{S}(\mathcal{A})\) can capture all singular values of \(\mathcal{A}\) from Figure 1.
(ii) The bounds of the largest singular value.
By Theorem 4 of [2], we have
Let \(S=\{1\}, \bar{S}=\{2,3\}\). By Theorem 2, we have
In fact, \(\lambda_{0}=31.1680\). This example shows that the bounds in Theorem 2 are better than those in Theorem 4 of [2].
Example 2
Let \(\mathcal{A}\in\mathbb{R}^{[2,2;2,2]}_{+}\) with entries defined as follows:
other \(a_{ijkl}=0\). By computation, we get that all different singular values of \(\mathcal {A}\) are \(0, 0.8226, 1, 3\).
(i) An S-type singular value inclusion set.
Let \(S=\{1\}\). Obviously, \(\bar{S}=\{2,3\}\). By Theorem 1, the S-type singular inclusion set is
The singular value inclusion set \(\Upsilon^{S}(\mathcal{A})\) and the exact singular values are drawn in Figure 2, where \(\Upsilon^{S}(\mathcal{A})\) is represented by black solid boundary and the exact singular values are plotted by red ‘+’. It is easy to see that \(\Upsilon^{S}(\mathcal{A})\) captures exactly all singular values of \(\mathcal{A}\) from Figure 2.
(ii) The bounds of the largest singular value.
By Theorem 2, we have
In fact, \(\lambda_{0}=3\). This example shows that the bounds in Theorem 2 are sharp.
4 Conclusions
In this paper, we give an S-type singular value inclusion set \(\Upsilon^{S}(\mathcal{A})\) for rectangular tensors. As an application of this set, an S-type upper bound \(U^{S}(\mathcal {A})\) and an S-type lower bound \(L^{S}(\mathcal{A})\) for the largest singular value \(\lambda_{0}\) of a nonnegative rectangular tensor \(\mathcal {A}\) are obtained and proved to be sharper than those in [2]. Then, an interesting problem is how to pick S to make \(\Upsilon ^{S}(\mathcal{A})\) as tight as possible. But it is difficult when the dimension of the tensor \(\mathcal{A}\) is large. We will continue to study this problem in the future.
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Acknowledgements
The author is very indebted to the reviewers for their valuable comments and corrections, which improved the original manuscript of this paper. This work is supported by Foundation of Guizhou Science and Technology Department (Grant No. [2015]2073), National Natural Science Foundation of China (Grant No. 11501141) and Natural Science Programs of Education Department of Guizhou Province (Grant No. [2016]066).
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Sang, C. An S-type singular value inclusion set for rectangular tensors. J Inequal Appl 2017, 141 (2017). https://doi.org/10.1186/s13660-017-1421-0
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DOI: https://doi.org/10.1186/s13660-017-1421-0