Polynomial SOS

This article is about representing a non-negative 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 of degree m such that

Every form that is SOS is also a positive polynomial, and although the converse is not always true, Hilbert proved that for n = 2, 2m = 2, or n = 3 and 2m = 4 a form is SOS if and only if it is positive.[1] The same is also valid for the analog problem on positive symmetric forms.[2][3]

Although not every form can be represented as SOS, explicit sufficient conditions for a form to be SOS have been found.[4][5] Moreover, every real nonnegative form can be approximated as closely as desired (in the -norm of its coefficient vector) by a sequence of forms that are SOS.[6]

  1. ^ Hilbert, David (September 1888). "Ueber die Darstellung definiter Formen als Summe von Formenquadraten". Mathematische Annalen. 32 (3): 342–350. doi:10.1007/bf01443605. S2CID 177804714.
  2. ^ Choi, M. D.; Lam, T. Y. (1977). "An old question of Hilbert". Queen's Papers in Pure and Applied Mathematics. 46: 385–405.
  3. ^ Goel, Charu; Kuhlmann, Salma; Reznick, Bruce (May 2016). "On the Choi–Lam analogue of Hilbert's 1888 theorem for symmetric forms". Linear Algebra and Its Applications. 496: 114–120. arXiv:1505.08145. doi:10.1016/j.laa.2016.01.024. S2CID 17579200.
  4. ^ Lasserre, Jean B. (2007). "Sufficient conditions for a real polynomial to be a sum of squares". Archiv der Mathematik. 89 (5): 390–398. arXiv:math/0612358. CiteSeerX 10.1.1.240.4438. doi:10.1007/s00013-007-2251-y. S2CID 9319455.
  5. ^ Powers, Victoria; Wörmann, Thorsten (1998). "An algorithm for sums of squares of real polynomials" (PDF). Journal of Pure and Applied Algebra. 127 (1): 99–104. doi:10.1016/S0022-4049(97)83827-3.
  6. ^ Lasserre, Jean B. (2007). "A Sum of Squares Approximation of Nonnegative Polynomials". SIAM Review. 49 (4): 651–669. arXiv:math/0412398. Bibcode:2007SIAMR..49..651L. doi:10.1137/070693709.