2 research outputs found
A Triangle Algorithm for Semidefinite Version of Convex Hull Membership Problem
Given a subset of , the set of
real symmetric matrices, we define its {\it spectrahull} as the
set , where is the {\it spectraplex}, . We let {\it spectrahull
membership} (SHM) to be the problem of testing if a given
lies in . On the one hand when 's are diagonal matrices,
SHM reduces to the {\it convex hull membership} (CHM), a fundamental problem in
LP. On the other hand, a bounded SDP feasibility is reducible to SHM. By
building on the {\it Triangle Algorithm} (TA) \cite{kalchar,kalsep}, developed
for CHM and its generalization, we design a TA for SHM, where given
, in iterations it either computes a
hyperplane separating from , or such that , maximum error over . Under certain conditions
iteration complexity improves to or even . The worst-case complexity of each iteration is , plus
testing the existence of a pivot, shown to be equivalent to estimating the
least eigenvalue of a symmetric matrix. This together with a semidefinite
version of Carath\'eodory theorem allow implementing TA as if solving a CHM,
resorting to the {\it power method} only as needed, thereby improving the
complexity of iterations. The proposed Triangle Algorithm for SHM is simple,
practical and applicable to general SDP feasibility and optimization. Also, it
extends to a spectral analogue of SVM for separation of two spectrahulls.Comment: 18 page
On the Equivalence of SDP Feasibility and a Convex Hull Relaxation for System of Quadratic Equations
We show {\it semidefinite programming} (SDP) feasibility problem is
equivalent to solving a {\it convex hull relaxation} (CHR) for a finite system
of quadratic equations. On the one hand, this offers a simple description of
SDP. On the other hand, this equivalence makes it possible to describe a
version of the {\it Triangle Algorithm} for SDP feasibility based on solving
CHR. Specifically, the Triangle Algorithm either computes an approximation to
the least-norm feasible solution of SDP, or using its {\it distance duality},
provides a separation when no solution within a prescribed norm exists. The
worst-case complexity of each iteration is computing the largest eigenvalue of
a symmetric matrix arising in that iteration. Alternate complexity bounds on
the total number of iterations can be derived. The Triangle Algorithm thus
provides an alternative to the existing interior-point algorithms for SDP
feasibility and SDP optimization. In particular, based on a preliminary
computational result, we can efficiently solve SDP relaxation of {\it binary
quadratic} feasibility via the Triangle Algorithm. This finds application in
solving SDP relaxation of MAX-CUT. We also show in the case of testing the
feasibility of a system of convex quadratic inequalities, the problem is
reducible to a corresponding CHR, where the worst-case complexity of each
iteration via the Triangle Algorithm is solving a {\it trust region
subproblem}. Gaining from these results, we discuss potential extension of CHR
and the Triangle Algorithm to solving general system of polynomial equations.Comment: 9 page