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Solving system of bilinear equations

MathOverflow Asked by grok on November 9, 2021

Consider a collection of $m$ matrices $A_i$ of size $ntimes n$, and a vector $b$ of size $m$. I want to solve the bilinear system

$$left{ x^T A_i y = b_i : i = 1,dots,m right}$$

in variables $x,y$. Is there an efficient way of doing this?

This is both a theoretical and a practical question: the matrices $A_i$ I have in mind are sparse and have size in the thousands. I understand that if $m=1$ then one can consider a vector $z=[x y]$ and run a semidefinite solver on $|z^T [0; A_1;A_1^T; 0]z-2b_1|$; however the $m$ I have in mind is also in the thousands.

One Answer

We have a system of $m$ bilinear equations in $mathrm x, mathrm y in mathbb R^n$

$$begin{aligned} mathrm x^top mathrm A_1 ,mathrm y &= b_1\ mathrm x^top mathrm A_2 ,mathrm y &= b_2\ &vdots\ mathrm x^top mathrm A_m ,mathrm y &= b_mend{aligned}$$

If matrices $mathrm A_1, mathrm A_2, dots, mathrm A_m$ are very sparse, perhaps it would not be utterly hopeless to use symbolic methods (e.g., Gröbner bases). However, we can use numerical methods. Note that

$$b_i = mathrm x^top mathrm A_i ,mathrm y = mbox{tr} left( mathrm x^top mathrm A_i ,mathrm y right) = mbox{tr} left( mathrm y^top mathrm A_i^top ,mathrm x right) = mbox{tr} left( mathrm A_i^top mathrm x mathrm y^top right) =: langle mathrm A_i, mathrm x mathrm y^toprangle$$

Let $mathrm Z := mathrm x mathrm y^top$. Hence, we have $m$ linear equality constraints in $rm Z$ and a constraint on its rank

$$begin{aligned} langle mathrm A_1, mathrm Z rangle &= b_1\ langle mathrm A_2, mathrm Zrangle &= b_2\ &vdots\ langle mathrm A_m, mathrm Zrangle &= b_m\ mbox{rank} (mathrm Z) &= 1end{aligned}$$

Since the nuclear norm is a convex proxy for the rank, we solve the following convex program in $rm Z$

$$begin{array}{ll} text{minimize} & | mathrm Z |_*\ text{subject to} & langle mathrm A_1, mathrm Z rangle = b_1\ & langle mathrm A_2, mathrm Z rangle = b_2\ & qquadvdots\ & langle mathrm A_m, mathrm Z rangle = b_mend{array}$$

If the optimal solution is rank-$1$, then we have solved the original system of $m$ bilinear equations.

Answered by Rodrigo de Azevedo on November 9, 2021

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