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

A056327

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

Terms

    a(0) =0a(1) =0a(2) =1a(3) =4a(4) =15a(5) =50a(6) =160a(7) =502a(8) =1545a(9) =4730a(10) =14356a(11) =43474a(12) =131145a(13) =395150a(14) =1188580a(15) =3572902a(16) =10732065a(17) =32225810a(18) =96733636a(19) =290322394a(20) =871200825a(21) =2614097750a(22) =7843255300a(23) =23531775502a(24) =70599259185a(25) =211805902490

External references