SIGN REGULAR MATRICES AND VARIATION DIMINUTION: SINGLE-VECTOR TESTS AND CHARACTERIZATIONS, FOLLOWING SCHOENBERG, GANTMACHER-KREIN, AND MOTZKIN

Variation diminution (VD) is a fundamental property in total positivity theory, first studied in 1912 by Fekete-Pólya for one-sided Pólya frequency sequences, followed by Schoenberg, and by Motzkin who characterized sign regular (SR) matrices using VD and some rank hypotheses. A classical theorem by Gantmacher-Krein characterized the strictly sign regular (SSR) m × n matrices for m > n using this property. In this article we strengthen these results by characterizing all m × n SSR matrices using VD. We further characterize strict sign regularity of a given sign pattern in terms of VD together with a natural condition motivated by total positivity. We then refine Motzkin's characterization of SR matrices by omitting the rank condition and specifying the sign pattern. This concludes a line of investigation on VD started by Fekete-Pólya [Rend. Circ. Mat. Palermo 34 (1912), pp. 89-120] and continued by Schoenberg [Math. Z. 32 (1930), pp. 321-328], Motzkin [Beiträge zur Theorie der linearen Ungleichungen, PhD Dissertation, Jerusalem, 1936], Gantmacher-Krein [Oscillyacionye matricy i yadra i malye kolebaniya mehaniceskih sistem, Gosudarstv. Izdat. Tehn.-Teor. Lit., Moscow-Leningrad, 1950], Brown-Johnstone-MacGibbon [J. Amer. Statist. Assoc. 76 (1981), pp. 824-832], and Choudhury [Bull. Lond. Math. Soc. 54 (2022), pp. 791-811; Bull. Sci. Math. 186 (2023), p. 21]. In fact we show stronger characterizations, by employing single test vectors with alternating sign coordinates - i.e., lying in the alternating bi-orthant. We also show that test vectors chosen from any other orthant will not work.