Constancy of the Hilbert-Samuel function

Abstract

The Hilbert-Samuel function and the multiplicity function are fundamental locally defined invariants on Noetherian schemes. They have been playing an important role in desingularization for many years. Bennett studied upper semicontinuity of the Hilbert-Samuel function on schemes and proved that it is non increasing under permissible blowing ups. The latter are blowing ups at regular subschemes along which the singular scheme is normally flat. For a reduced scheme, the Hilbert-Samuel function is constant if and only if it is regular: this translates the question of resolution of singularities into a problem of lowering the Hilbert-Samuel function. We show here that this result can be extended to non reduced schemes as follows: Given a locally Noetherian scheme X such that the local rings are excellent for every point, then the Hilbert-Samuel function is constant on X if and only if X is normally flat along its reduction and the reduction itself is regular.