A068042 Number of subsets of {1,2,3,...,n} that sum to 0 mod 21.

1, 1, 1, 1, 1, 1, 2, 6, 14, 26, 49, 99, 197, 391, 782, 1564, 3123, 6244, 12490, 24970, 49940, 99880, 199738, 399472, 798940, 1597844, 3195684, 6391368, 12782668, 25565336, 51130664, 102261174, 204522354, 409044702, 818089110, 1636178220