a(n) is also the number of multisets of integers ranging from 1 to n, such that the sum of the members of the multiset is congruent to 0 mod n, and no submultiset exists whose sum of members is congruent to 0 mod n. These multisets can be thought of as partitions of n in modular arithmetic, thus this sequence can be thought of as a modular arithmetic version of the partition numbers (cf. A000041). - Andrew Weimholt, Jan 31 2011
REFERENCES
M. D. Neusel and L. Smith, Invariant Theory of Finite Groups, Amer. Math. Soc., 2002; see p. 208.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
C. W. Strom, Complete systems of invariants of the cyclic groups of equal order and degree, Proc. Iowa Acad. Sci., 55 (1948), 287-290.