Ideals Generated by Quadratic Polynomials

Abstract

Let $R$ be a polynomial ring in $N$ variables over an arbitrary field $K$ and let $I$ be an ideal of $R$ generated by $n$ polynomials of degree at most 2. We show that there is a bound on the projective dimension of $R/I$ that depends only on $n$, and not on $N$. The proof depends on showing that if $K$ is infinite and $n$ is a positive integer, there exists a positive integer C(n), independent of $N$, such that any $n$ forms of degree at most 2 in $R$ are contained in a subring of $R$ generated over $K$ by at most $t \leq C(n)$ forms $G_1, \,..., \, G_t$ of degree 1 or 2 such that $G_1, \,..., \, G_t$ is a regular sequence in $R$. C(n) is asymptotic to $2n^{2n}$.