Additive Splittings of Homogeneous Polynomials

Abstract: In this thesis we study when a homogeneous polynomial $f$ decomposes or "splits" additively. Up to base change this means that it is possible to write $f = g + h$ where $g$ and $h$ are polynomials in independent sets of variables. This simple idea leads us to define a set $M_f$ of matrices associated to $f$. Surprisingly, $M_f$ turns out to be a commutative matrix algebra when $deg f \ge 3$. We show how all (regular) splittings $f = g_1 + ... + g_n$ can be computed from $M_f$. Next we show how to find the minimal free resolution of the graded Artinian Gorenstein quotient $R/\ann f$, assuming the minimal free resolutions of its additive components $R/\ann g_i$ are known. From this we get simple formulas for the Hilbert function $H$ and the graded Betti numbers of $R/\ann f$. We may use this to compute the dimension of a "splitting subfamily" of the parameter space $\PGor (H)$. Its closure is quite often an irreducible component of $\PGor (H)$. We will also study degenerations of polynomials that split and see how they relate to $M_f$. This situation is more difficult, but we are able to prove several partial results that together cover many interesting cases. In particular, we prove that $f$ has a regular or degenerate splitting if and only if the ideal $\ann f$ has at least one generator in its socle degree. Finally, we look at some generalizations of $M_f$.