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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =2a(7) =6a(8) =14a(9) =26a(10) =49a(11) =99a(12) =197a(13) =391a(14) =782a(15) =1564a(16) =3123a(17) =6244a(18) =12490a(19) =24970a(20) =49940a(21) =99880a(22) =199738a(23) =399472a(24) =798940a(25) =1597844a(26) =3195684a(27) =6391368a(28) =12782668a(29) =25565336