Number of palindromic structures using a maximum of five different symbols.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =5a(5) =5a(6) =15a(7) =15a(8) =52a(9) =52a(10) =202a(11) =202a(12) =855a(13) =855a(14) =3845a(15) =3845a(16) =18002a(17) =18002a(18) =86472a(19) =86472a(20) =422005a(21) =422005a(22) =2079475a(23) =2079475a(24) =10306752a(25) =10306752a(26) =51263942a(27) =51263942a(28) =255514355a(29) =255514355