Integral orthogonal bases of small height for real polynomial spaces

Lenny Fukshansky

Abstract


Let $\mathcal P_N(\mathbb R)$ be the space of all real polynomials in $N$ variables with the usual inner product $\langle ~,~ \rangle$ on it, given by integrating over the unit sphere. We start by deriving an explicit combinatorial formula for the bilinear form representing this inner product on the space of coefficient vectors of all polynomials in $\mathcal P_N(\mathbb R)$ of degree $\leq M$. We exhibit two applications of this formula. First, given a finite dimensional subspace $V$ of $\mathcal P_N(\mathbb R)$ defined over $\mathbb Q$, we prove the existence of an orthogonal basis for $(V, \langle ~,~ \rangle)$, consisting of polynomials of small height with integer coefficients, providing an explicit bound on the height; this can be viewed as a version of Siegel’s lemma for real polynomial inner product spaces. Secondly, we derive a criterion for a finite set of points on the unit sphere in $\mathbb R^N$ to be a spherical $M$-design.


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