Unitarity of four-particle superstring tree amplitudes implies that Hankel matrices built from low-energy coefficients are totally positive, yielding new inequalities involving multiple zeta values, including irreducible ones.
Total positivity of sums, Hadamard products and Hadamard powers: Results and counterexamples
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abstract
We show that, for Hankel matrices, total nonnegativity (resp. total positivity) of order r is preserved by sum, Hadamard product, and Hadamard power with real exponent t \ge r-2. We give examples to show that our results are sharp relative to matrix size and structure (general, symmetric or Hankel). Some of these examples also resolve the Hadamard critical-exponent problem for totally positive and totally nonnegative matrices.
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Superstring Amplitudes, Unitarity, and Hankel Determinants of Multiple Zeta Values
Unitarity of four-particle superstring tree amplitudes implies that Hankel matrices built from low-energy coefficients are totally positive, yielding new inequalities involving multiple zeta values, including irreducible ones.