Positive Polynomials on Compact Semi-Algebraic Sets Mihai Putinar Indiana University Mathematics Journal 1993 text pdf 10.1512/iumj.1993.42.42045 10.1512/iumj.1993.42.42045 en Indiana Univ. Math. J. 42 (1993) 969 - 984 state-of-the-art mathematics http://iumj.org/access/