On primary decomposition of Hermite projectors

Abstract

An ideal projector on the space of polynomials $\mathbb{C}[\mathbf{x}]=\mathbb{C}[x_1,\dots ,x_d]$ is a projector whose kernel is an ideal in $\mathbb{C}[\mathbf{x}]$. The question of characterization of ideal projectors that are limits of Lagrange projector was posed by Carl de Boor. In this paper we make a contribution to this problem. Every ideal projector $P$ can be written as a sum of ideal projector $\sum P^{(k)}$  such that $\cap \ker P^{(k)}$ is a primary decomposition of the ideal $\ker P$. We show that $P$ is a limit of Lagrange projectors if and only if each $P^{(k)}$ is. As an application we construct an ideal projector $P$ whose kernel is a symmetric ideal, yet $P$ is not a limit of Lagrange projectors.

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