Number of palindromic structures using exactly six different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =0a(10) =1a(11) =1a(12) =21a(13) =21a(14) =266a(15) =266a(16) =2646a(17) =2646a(18) =22827a(19) =22827a(20) =179487a(21) =179487a(22) =1323652a(23) =1323652a(24) =9321312a(25) =9321312a(26) =63436373a(27) =63436373a(28) =420693273a(29) =420693273