Infinite log-concavity: developments and conjectures

Details Infinite log-concavity: developments and conjectures Infinite log-concavity: developments and conjectures

2008-08-07

arxiv.org/abs/0808.1065v2

Advances in Applied Mathematics, 44 (1) (2010), 1-15

Mathematics

Combinatorics

21 pages. Minor changes and additional references. Final version, to appear in Advances in Applied Mathematics

Scientific paper

Given a sequence (a_k) = a_0, a_1, a_2,... of real numbers, define a new sequence L(a_k) = (b_k) where b_k = a_k^2 - a_{k-1} a_{k+1}. So (a_k) is log-concave if and only if (b_k) is a nonnegative sequence. Call (a_k) "infinitely log-concave" if L^i(a_k) is nonnegative for all i >= 1. Boros and Moll conjectured that the rows of Pascal's triangle are infinitely log-concave. Using a computer and a stronger version of log-concavity, we prove their conjecture for the nth row for all n <= 1450. We also use our methods to give a simple proof of a recent result of Uminsky and Yeats about regions of infinite log-concavity. We investigate related questions about the columns of Pascal's triangle, q-analogues, symmetric functions, real-rooted polynomials, and Toeplitz matrices. In addition, we offer several conjectures.

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