Computing the generators of the truncated real radical ideal by moment matrices and SDP facial reduction (1606.00491v3)

Abstract: Recent breakthroughs have been made in the use of semidefinite programming and its application to real polynomial solving. For example, the real radical of a zero dimensional ideal, can be determined by such approaches as shown by Lasserre and collaborators. Some progress has been made on the determination of the real radical in positive dimension by Ma, Wang and Zhi. Such work involves the determination of maximal rank semidefinite moment matrices. Existing methods are computationally expensive and have poorer accuracy on larger examples. This paper is motivated by problems in the numerical computation of the real radical ideal in the general positive case. In this paper we give a method to compute the generators of the real radical for any given degree $d$. We combine the use of moment matrices and techniques from SDP optimization: facial reduction first developed by Borwein and Wolkowicz. In use of the semidefinite moment matrices to compute the real radical, the maximum rank property is very key, and with facial reduction, it can be guaranteed with very high accuracy. Our algorithm can be used to test the real radical membership of a given polynomial. In a special situation, we can determine the real radical ideal in the positive dimensional case.