a(n) has generating function 1/((1-x)^k*(1-x^2)^(k*(k-1)/2)) for k=7.

Terms

a(0) =1a(1) =7a(2) =49a(3) =231a(4) =1029a(5) =3843a(6) =13573a(7) =43219a(8) =131131a(9) =370909a(10) =1007083a(11) =2596573a(12) =6466159a(13) =15465961a(14) =35906959a(15) =80682553a(16) =176682268a(17) =376497604a(18) =784435036a(19) =1596836164a(20) =3186750028a(21) =6232957588a(22) =11978020684a(23) =22615355476a(24) =42031123204a(25) =76900938268a(26) =138714560068a(27) =246728568604