Dirichlet polynomials and entropy

07/10/2021

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by   David I. Spivak, et al.

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A Dirichlet polynomial d in one variable 𝓎 is a function of the form d(𝓎)=a_n n^𝓎+⋯+a_22^𝓎+a_11^𝓎+a_00^𝓎 for some n,a_0,…,a_n∈ℕ. We will show how to think of a Dirichlet polynomial as a set-theoretic bundle, and thus as an empirical distribution. We can then consider the Shannon entropy H(d) of the corresponding probability distribution, and we define its length (or, classically, its perplexity) by L(d)=2^H(d). On the other hand, we will define a rig homomorphism h𝖣𝗂𝗋→𝖱𝖾𝖼𝗍 from the rig of Dirichlet polynomials to the so-called rectangle rig, whose underlying set is ℝ_≥0×ℝ_≥0 and whose additive structure involves the weighted geometric mean; we write h(d)=(A(d),W(d)), and call the two components area and width (respectively). The main result of this paper is the following: the rectangle-area formula A(d)=L(d)W(d) holds for any Dirichlet polynomial d. In other words, the entropy of an empirical distribution can be calculated entirely in terms of the homomorphism h applied to its corresponding Dirichlet polynomial. We also show that similar results hold for the cross entropy.