Number of reversible string structures with n beads using exactly four different colors.

A056328

Number of reversible string structures with n beads using exactly four different colors.

Terms

    a(0) =0a(1) =0a(2) =0a(3) =1a(4) =6a(5) =37a(6) =183a(7) =877a(8) =3930a(9) =17185a(10) =73095a(11) =306361a(12) =1267266a(13) =5198557a(14) =21182343a(15) =85910917a(16) =347187210a(17) =1399451545a(18) =5629911015a(19) =22616256721a(20) =90754855026a(21) =363890126677

External references