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Special Types of Linear Maps: Symmetric Tensors

Symmetric Tensor Definition

Let S:\mathbb{R}^n\rightarrow\mathbb{R}^n. S is called a symmetric tensor if S=S^T.
The following properties can be naturally deduced from the definition of symmetric tensors:

  • In component form, the matrix representation of S is such that S_{ij}=S_{ji}
  • \forall a,b\in\mathbb{R}^n we have:

    \[Sa\cdot b=a\cdot Sb\]

  • \forall M:\mathbb{R}^n\rightarrow\mathbb{R}^n:M^TSM is symmetric. In particular, if M is an orthogonal matrix associated with a coordinate transformation, then the matrix representation of S stays symmetric in any coordinate system.

Symmetric Tensors Possess Real Eigenvalues and an Orthonormal Set of Eigenvectors

Symmetric tensors form a very important class of tensors that appear in many engineering applications. The following assertion leads to the simplification of the study of symmetric tensors.


A tensor S:\mathbb{R}^n\rightarrow\mathbb{R}^n is symmetric if and only if it possesses n real eigenvalues associated with n orthonormal eigenvectors.


Let \mathbb{C} be the space of complex numbers. First we will recall a few facts from complex analysis:
If a\in\mathbb{C}, then, \exists a_1,a_2 \in \mathbb{R} such that a=a_1+a_2i where i^2=-1. The conjugate of a, namely \bar{a} is defined as \bar{a}=a_1-a_2i. The product a\bar{a}=a_1^2+a_2^2\in[0,\infty[. Additionally, \forall a,b\in\mathbb{C},\overline{ab}=\bar{a}\bar{b}.

If \lambda is an eigenvalue of S, then, it is a solution to the n^{th} degree polynomial equation \det(S-\lambda I)=0. In general, this equation has n solutions (roots) and some of these roots can be complex. We will now argue by contradiction: Assuming that \lambda is a complex eigenvalue of S, then \exists p\neq 0, p\in\mathbb{C}^3 such that:

    \[ Sp=\lambda p \]

Taking the conjugate of the above relation:

    \[ S\bar{p}=\overline{\lambda p}=\bar{\lambda}\bar{p} \]

If we then take the dot product of the first equation with \bar{p} and the second equation with p we get:

    \[ \bar{p}\cdot Sp=\lambda \bar{p}\cdot p\hspace{10mm}p\cdot S\bar{p}=\bar{\lambda} p\cdot \bar{p} \]

But, since S is symmetric we have:

    \[ \bar{p}\cdot Sp=p\cdot S\bar{p}\Rightarrow\lambda \bar{p}\cdot p-\bar{\lambda} p\cdot \bar{p}=(\lambda-\bar{\lambda}) p\cdot \bar{p}=0 \]

However, we have \forall p \in\mathbb{C}^3,p\neq 0: \bar{p}\cdot p>0. Therefore, \lambda=\bar{\lambda} and therefore, \lambda \in \mathbb{R}.

The next step is to show that we can find a set of n orthonormal eigenvectors associated with the n real eigenvalues of S. We first show this for any two distinct eigenvalues \lambda_1\neq\lambda_2. Let p_1 and and p_2 be the corresponding eigenvectors, then:

    \[ p_1\cdot Sp_2=p_2\cdot Sp_1=\lambda_1(p_1\cdot p_2)=\lambda_2(p_1\cdot p_2)\Rightarrow(\lambda_1-\lambda_2)(p_1\cdot p_2)=0 \]

But since \lambda_1\neq\lambda_2, then p_1 is orthogonal to p_2.

Next, we assume that there is an eigenvalue \lambda_1 with multiplicity m\geq2, i.e.,

(1)   \begin{equation*} \det(S-\lambda I)=(\lambda_1-\lambda)^m f(\lambda^{n-m}) \end{equation*}

where f is a real valued function. We need to show that there is a set of orthogonal (linearly independent) vectors p_i, i\leq m associated with the eigenvalue \lambda_1.
Let p_1 be an eigenvector associated with \lambda_1. We can choose a set of vectors y_i, 2\leq i\leq n that forms with p_1 an orthonormal basis set. Then, the following is the coordinate transformation matrix to change from the basis set B=\{e_1,e_2,\cdots,e_n\} to B'=\{p_1,y_2,y_3,\cdots,y_n\}:

    \[ Q=\left( \begin{array}{cccc} p_{1,1}&p_{1,2}&\cdots&p_{1,n}\\ y_{2,1}&y_{2,2}&\cdots&y_{2,n}\\ \vdots&\vdots&\ddots&\vdots\\ y_{n,1}&y_{n,2}&\cdots&y_{n,n} \end{array} \right) \]

If S' is the matrix representation of S in the coordinate system of B', then the components of S' can be evaluated as follows:

    \[ S'=QSQ^T= \left( \begin{array}{cccc} p_{1,1}&p_{1,2}&\cdots&p_{1,n}\\ y_{2,1}&y_{2,2}&\cdots&y_{2,n}\\ \vdots&\vdots&\ddots&\vdots\\ y_{n,1}&y_{n,2}&\cdots&y_{n,n} \end{array} \right) S \left( \begin{array}{cccc} p_{1,1}&y_{2,1}&\cdots&y_{n,1}\\ p_{1,2}&y_{2,2}&\cdots&y_{2,n}\\ \vdots&\vdots&\ddots&\vdots\\ p_{1,n}&y_{2,n}&\cdots&y_{n,n} \end{array} \right) \]


    \[\begin{split} S'&=\left( \begin{array}{cccc} p_{1,1}&p_{1,2}&\cdots&p_{1,n}\\ y_{2,1}&y_{2,2}&\cdots&y_{2,n}\\ \vdots&\vdots&\ddots&\vdots\\ y_{n,1}&y_{n,2}&\cdots&y_{n,n} \end{array} \right) \left( \begin{array}{cccc} \vdots&\vdots&\cdots&\vdots\\ Sp_1&Sy_2&\cdots&Sy_n\\ \vdots&\vdots&\cdots&\vdots\\ \end{array} \right)\\ & = \left( \begin{array}{cccc} p_1\cdot Sp_1&p_1\cdot Sy_2&\cdots&p_1\cdot Sy_n\\ y_2\cdot Sp_1&y_2\cdot Sy_2&\cdots&y_2\cdot Sy_n\\ \vdots&\vdots&\ddots&\vdots\\ y_n\cdot Sp_1&y_n\cdot Sy_2&\cdots&y_n\cdot Sy_n \end{array} \right) \end{split} \]

Notice that p_1\cdot Sp_1=\lambda_1 and \forall i:p_1\cdot Sy_i=Sp_1\cdot y_i=\lambda_1 (p_1\cdot y_i) = 0. Therefore:

    \[ S'= \left( \begin{array}{cccc} \lambda_1&0&\cdots&0\\ 0&y_2\cdot Sy_2&\cdots&y_2\cdot Sy_n\\ \vdots&\vdots&\ddots&\vdots\\ 0&y_n\cdot Sy_2&\cdots&y_n\cdot Sy_n \end{array} \right) \]

The above expression can be simplified if we consider the following matrix:

    \[ Y=\left( \begin{array}{cccc} y_{2,1}&y_{2,2}&\cdots&y_{2,n}\\ y_{3,1}&y_{3,2}&\cdots&y_{3,n}\\ \vdots&\vdots&\ddots&\vdots\\ y_{n,1}&y_{n,2}&\cdots&y_{n,n} \end{array} \right) \]

Then, S' can be written in the following simplified form:

    \[ S'= \left( \begin{array}{cc} \lambda_1&0\\ 0&YSY^T \end{array} \right) \]

From the properties of the determinant, the characteristic equation of S is:

    \[ \det(S-\lambda I)=\det(S'-\lambda I)=(\lambda_1-\lambda)\det(YSY^T-\lambda I_{n-1}) \]

Where I_{n-1} is the identity matrix defined on the subspace of the basis vectors y_2, y_3,\cdots,y_n. It follows from (1) that \lambda_1 is an eigenvalue for the matrix YSY^T, i.e., it is an eigenvalue of S. Let p_2 be the corresponding eigenvector, then p_2\in \text{span}\{y_2, y_3,\cdots,y_n\}. Therefore, p_2\cdot p_1=0. Therefore, \lambda_1 has at least two perpendicular eigenvectors. By repeating the above argument, we can find m orthogonal eigenvectors associated with \lambda_1. Note that it is not possible to find more than m orthogonal eigenvectors associated with \lambda_1, otherwise this would lead to the contradictory conclusion that there are more than n orthogonal eigenvectors in \mathbb{R}^n.

Note that the choice of the eigenvectors is not necessarily unique since if p is an eigenvector of S, then so is \alpha p for all nonzero real numbers \alpha. In addition, if p and q are two orthogonal eigenvectors associated with an eigenvalue \lambda_1, then there are infinite choices of sets of two orthonormal vectors associated with the eigenvalue \lambda_1 (why?).

It is now left to show that if S has n real eigenvalues associated with n orthonormal eigenvectors, then S is symmetric. Let \{p_i\}_{i=1}^n be the set of n orthonormal eigenvectors associated (respectively) with the the set of eigenvalues \{\lambda_i\}_{i=1}^n, then \forall x,y\in\mathbb{R}^n, x=\sum_{i=1}^n (x\cdot p_i) p_i and y=\sum_{i=1}^n (y\cdot p_i) p_i. Therefore:

    \[ x\cdot Sy=\left(\sum_{i=1}^n (x\cdot p_i) p_i\right)\cdot S\left(\sum_{j=1}^n (y\cdot p_j) p_j\right)=\sum_{i=1}^n \lambda_i(y\cdot p_i)(x\cdot p_i) \]


    \[ y\cdot Sx=\sum_{i=1}^n \lambda_i(y\cdot p_i)(x\cdot p_i) \]

Therefore, S is symmetric.


Representation of Symmetric Tensors in \mathbb{R}^3

From the previous section, S possesses three perpendicular eigenvectors, say p, q and r associated with the eigenvalues \lambda_p, \lambda_q and \lambda_r respectively. Assuming that p, q and r form a right handed orthonormal basis set in \mathbb{R}^3 then, \forall a\in\mathbb{R}^3:a=(a\cdot p)p+(a\cdot q)q+(a\cdot r)r. Then:

    \[\begin{split} Sa&=(a\cdot p)Sp+(a\cdot q)Sq+(a\cdot r)Sr=\lambda_p(a\cdot p)p+\lambda_q(a\cdot q)q+\lambda_r(a\cdot r)r\\ &=\left(\lambda_p(p\otimes p)+\lambda_q(q\otimes q)+\lambda_r(r\otimes r)\right)a \end{split} \]

Therefore, S admits the representation:

(2)   \begin{equation*} S=\lambda_p(p\otimes p)+\lambda_q(q\otimes q)+\lambda_r(r\otimes r) \end{equation*}

Note that this representation is not restricted to \mathbb{R}^3 but can be extended to any finite dimensional vector space \mathbb{R}^n.

Diagonalization of Symmetric Matrices in \mathbb{R}^3

The representation of a symmetric tensor shown in (2) implies that if a coordinate system of the eigenvectors p, q and r is chosen, then S admits a diagonal matrix representation.
This is easy to see when we consider a coordinate transformation from the basis set B=\{e_1,e_2,e_3\} to the basis set B'=\{p,q,r\}. The matrix of transformation has the form:

    \[ Q=\left(\begin{array}{ccc} p_1 & p_2&p_3\\ q_1&q_2&q_3\\r_1&r_2&r_3\end{array}\right) \]

If S' is the representation of S in the coordinate system described by p, q and r then:

    \[ S'=QSQ^T=\left(\begin{array}{ccc} p_1 & p_2&p_3\\ q_1&q_2&q_3\\r_1&r_2&r_3\end{array}\right) \left(\begin{array}{ccc} S_{11} & S_{12}&S_{13}\\ S_{21} & S_{22}&S_{23}\\S_{31} & S_{32}&S_{33}\end{array}\right) \left(\begin{array}{ccc} p_1 & q_1&r_1\\ p_2&q_2&r_2\\p_3&q_3&r_3\end{array}\right) \]

Utilizing the identities: \sum_{j=1}^3S_{ij}p_j=\lambda_p p_i, \sum_{j=1}^3S_{ij}q_j=\lambda_q q_i and \sum_{j=1}^3S_{ij}r_j=\lambda_r r_i:

    \[ S'=\left(\begin{array}{ccc} p_1 & p_2&p_3\\ q_1&q_2&q_3\\r_1&r_2&r_3\end{array}\right) \left(\begin{array}{ccc} \lambda_p p_1 & \lambda_q q_1&\lambda_r r_1\\ \lambda_p p_2&\lambda_q q_2&\lambda_r r_2\\\lambda_p p_3&\lambda_q q_3&\lambda_r r_3\end{array}\right) \]

Since p\cdot p = \sum_{i=1}^3 p_i p_i=q\cdot q = \sum_{i=1}^3 q_i q_i=r\cdot r = \sum_{i=1}^3 r_i r_i=1 and p\cdot q=p\cdot r=q\cdot r=0 then:

    \[ S'=\left(\begin{array}{ccc} \lambda_p & 0&0\\ 0&\lambda_q&0\\0&0&\lambda_r\end{array}\right) \]

Note that this representation is not restricted to \mathbb{R}^3 but can be extended to any finite dimensional vector space \mathbb{R}^n.

Principal Invariants of a Symmetric Matrix in \mathbb{R}^3

The diagonalization described in the previous section of a symmetric matrix allows expressing the three principal invariants of a symmetric matrix S in terms of the three eigenvalues \lambda_p, \lambda_q and \lambda_r as follows:

    \[ I_1(S) = \lambda_p+\lambda_q+\lambda_r \]

    \[ I_2(S) = \lambda_p\lambda_q+\lambda_p\lambda_r+\lambda_q\lambda_r \]

    \[ I_3(S) = \lambda_p\lambda_q\lambda_r \]

Change the entries for the components of the symmetric matrix S:\mathbb{R}^2\rightarrow\mathbb{R}^2 and the tool will find the eigenvalues, eigenvectors and the new coordinate system in which S is diagonal. The coordinate system used is illustrated with thick arrows:

Change the entries for the components of the symmetric matrix S:\mathbb{R}^3\rightarrow\mathbb{R}^3 and the tool will find the eigenvalues, eigenvectors and the new coordinate system in which S is diagonal:


Geometric Representation of Symmetric Matrices

The geometric function of a symmetric matrix S is to stretch an object along the principal direction (eigenvectors) of the matrix.

The following example illustrates the action of a symmetric matrix on the vectors forming a circle of radius 1 to transform the circle into an ellipse with major and minor radii equal to the eigenvalues of S. The blue and red arrows show the eigenvectors of S which upon transformation, do not change direction but change their length according to the corresponding eigenvalues. The black solid arrows show the vectors e_1 and Se_1 while the black dotted arrows show the vectors e_2 and Se_2. Notice what happens to the orientation of the vectors when \det(S)<0. Can you also find a combination of components producing \det(S)=0? What happens to the transformed circle?

Similarly, the following example illustrates the action of a symmetric tensor on the vectors forming a sphere of radius 1 to transform the sphere into an ellipsoid with radii equal to the eigenvalues of S.


Positive Definite and Semi-Positive Definite Symmetric Tensors

A symmetric tensor S:\mathbb{R}^n\rightarrow\mathbb{R}^n is positive definite if \forall a\in\mathbb{R}^n,a\neq 0:a\cdot Sa>0.

A symmetric tensor S:\mathbb{R}^n\rightarrow\mathbb{R}^n is semi-positive definite if \forall a\in\mathbb{R}^n:a\cdot Sa\geq 0.


Let S:\mathbb{R}^n\rightarrow\mathbb{R}^n be symmetric, then S is positive (semi-positive) definite if and only if \forall \lambda in the set of eigenvalues of S: \lambda>0 (\lambda\geq 0)


Left as an exercise

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