  The Hilbert scheme H^d_n of n points in A^d contains an irreducible component
R^d_n which generically represents n distinct points in A^d. We show that when
n is at most 8, the Hilbert scheme H^d_n is reducible if and only if n = 8 and
d >= 4. In the simplest case of reducibility, the component R^4_8 \subset H^4_8
is defined by a single explicit equation which serves as a criterion for
deciding whether a given ideal is a limit of distinct points.
  To understand the components of the Hilbert scheme, we study the closed
subschemes of H_n^d which parametrize those ideals which are homogeneous and
have a fixed Hilbert function. These subschemes are a special case of
multigraded Hilbert schemes, and we describe their components when the colength
is at most 8. In particular, we show that the scheme corresponding to the
Hilbert function (1,3,2,1) is the minimal reducible example.

Algebraic Geometry;Commutative Algebra