Sequence S_6 of the S_r family.

Terms

a(0) =0a(1) =1a(2) =6a(3) =25a(4) =96a(5) =361a(6) =1350a(7) =5041a(8) =18816a(9) =70225a(10) =262086a(11) =978121a(12) =3650400a(13) =13623481a(14) =50843526a(15) =189750625a(16) =708158976a(17) =2642885281a(18) =9863382150a(19) =36810643321a(20) =137379191136a(21) =512706121225