Polynomial SOS

This article is about representing polynomial as sum of squares of polynomials. For representing polynomial as a sum of squares of rational functions, see Hilbert's seventeenth problem. For the sum of squares of consecutive integers, see square pyramidal number. For representing an integer as a sum of squares of integers, see Lagrange's four-square theorem.

In mathematics, a form (i.e. a homogeneous polynomial) h(x) of degree 2m in the real n-dimensional vector x is sum of squares of forms (SOS) if and only if there exist forms $g_1(x),\ldots,g_k(x)$ of degree m such that

$h(x)=\sum_{i=1}^k g_i(x)^2 .$

Explicit sufficient conditions for a form to be SOS have been found. However every real nonnegative form can be approximated as closely as desired (in the $l_1$-norm of its coefficient vector) by a sequence of forms $\{f_\epsilon\}$ that are SOS.

Square matricial representation (SMR)

To establish whether a form h(x) is SOS amounts to solving a convex optimization problem. Indeed, any h(x) can be written as

$h(x)=x^{\{m\}'}\left(H+L(\alpha)\right)x^{\{m\}}$

where $x^{\{m\}}$ is a vector containing a base for the forms of degree m in x (such as all monomials of degree m in x), the prime ′ denotes the transpose, H is any symmetric matrix satisfying

$h(x)=x^{\left\{m\right\}'}Hx^{\{m\}}$

and $L(\alpha)$ is a linear parameterization of the linear space

$\mathcal{L}=\left\{L=L':~x^{\{m\}'} L x^{\{m\}}=0\right\}.$

The dimension of the vector $x^{\{m\}}$ is given by

$\sigma(n,m)=\binom{n+m-1}{m}$

whereas the dimension of the vector $\alpha$ is given by

$\omega(n,2m)=\frac{1}{2}\sigma(n,m)\left(1+\sigma(n,m)\right)-\sigma(n,2m).$

Then, h(x) is SOS if and only if there exists a vector $\alpha$ such that

$H + L(\alpha) \ge 0,$

meaning that the matrix $H + L(\alpha)$ is positive-semidefinite. This is a linear matrix inequality (LMI) feasibility test, which is a convex optimization problem. The expression $h(x)=x^{\{m\}'}\left(H+L(\alpha)\right)x^{\{m\}}$ was introduced in with the name square matricial representation (SMR) in order to establish whether a form is SOS via an LMI. This representation is also known as Gram matrix.

Examples

• Consider the form of degree 4 in two variables $h(x)=x_1^4-x_1^2x_2^2+x_2^4$. We have

$m=2,~x^{\{m\}}=\left(\begin{array}{c}x_1^2\\x_1x_2\\x_2^2\end{array}\right), ~H+L(\alpha)=\left(\begin{array}{ccc} 1&0&-\alpha_1\\0&-1+2\alpha_1&0\\-\alpha_1&0&1 \end{array}\right).$

Since there exists α such that $H+L(\alpha)\ge 0$, namely $\alpha=1$, it follows that h(x) is SOS.

• Consider the form of degree 4 in three variables $h(x)=2x_1^4-2.5x_1^3x_2+x_1^2x_2x_3-2x_1x_3^3+5x_2^4+x_3^4$. We have

$m=2,~x^{\{m\}}=\left(\begin{array}{c}x_1^2\\x_1x_2\\x_1x_3\\x_2^2\\x_2x_3\\x_3^2\end{array}\right), ~H+L(\alpha)=\left(\begin{array}{cccccc} 2&-1.25&0&-\alpha_1&-\alpha_2&-\alpha_3\\ -1.25&2\alpha_1&0.5+\alpha_2&0&-\alpha_4&-\alpha_5\\ 0&0.5+\alpha_2&2\alpha_3&\alpha_4&\alpha_5&-1\\ -\alpha_1&0&\alpha_4&5&0&-\alpha_6\\ -\alpha_2&-\alpha_4&\alpha_5&0&2\alpha_6&0\\ -\alpha_3&-\alpha_5&-1&-\alpha_6&0&1 \end{array}\right).$

Since $H+L(\alpha)\ge 0$ for $\alpha=(1.18,-0.43,0.73,1.13,-0.37,0.57)$, it follows that h(x) is SOS.

Generalizations

Matrix SOS

A matrix form F(x) (i.e., a matrix whose entries are forms) of dimension r and degree 2m in the real n-dimensional vector x is SOS if and only if there exist matrix forms $G_1(x),\ldots,G_k(x)$ of degree m such that

$F(x)=\sum_{i=1}^k G_i(x)'G_i(x) .$

Matrix SMR

To establish whether a matrix form F(x) is SOS amounts to solving a convex optimization problem. Indeed, similarly to the scalar case any F(x) can be written according to the SMR as

$F(x)=\left(x^{\{m\}}\otimes I_r\right)'\left(H+L(\alpha)\right)\left(x^{\{m\}}\otimes I_r\right)$

where $\otimes$ is the Kronecker product of matrices, H is any symmetric matrix satisfying

$F(x)=\left(x^{\{m\}}\otimes I_r\right)'H\left(x^{\{m\}}\otimes I_r\right)$

and $L(\alpha)$ is a linear parameterization of the linear space

$\mathcal{L}=\left\{L=L':~\left(x^{\{m\}}\otimes I_r\right)'L\left(x^{\{m\}}\otimes I_r\right)=0\right\}.$

The dimension of the vector $\alpha$ is given by

$\omega(n,2m,r)=\frac{1}{2}r\left(\sigma(n,m)\left(r\sigma(n,m)+1\right)-(r+1)\sigma(n,2m)\right).$

Then, F(x) is SOS if and only if there exists a vector $\alpha$ such that the following LMI holds:

$H+L(\alpha) \ge 0.$

The expression $F(x)=\left(x^{\{m\}}\otimes I_r\right)'\left(H+L(\alpha)\right)\left(x^{\{m\}}\otimes I_r\right)$ was introduced in in order to establish whether a matrix form is SOS via an LMI.

Noncommutative polynomial SOS

Consider the free algebra R⟨X⟩ generated by the n noncommuting letters X = (X₁,...,X_n) and equipped with the involution ^T, such that ^T fixes R and X₁,...,X_n and reverse words formed by X₁,...,X_n. By analogy with the commutative case, the noncommutative symmetric polynomials f are the noncommutative polynomials of the form f=f^T. When any real matrix of any dimension r x r is evaluated at a symmetric noncommutative polynomial f results in a positive semi-definite matrix, f is said to be matrix-positive.

A noncommutative polynomial is SOS if there exists noncommutative polynomials $h_1,\ldots,h_k$ such that

$f(X) = \sum_{i=1}^{k} h_i(X)^T h_i(X).$

Surprisingly, in the noncommutative scenario a noncommutative polynomial is SoS if and only if is matrix-positive. Moreover, there exist algorithms available to decompose matrix-positive polynomials in sum of squares of noncommutative polynomials.

See also

• Sum-of-squares optimization