Number of palindromic structures using a maximum of six 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) =203a(11) =203a(12) =876a(13) =876a(14) =4111a(15) =4111a(16) =20648a(17) =20648a(18) =109299a(19) =109299a(20) =601492a(21) =601492a(22) =3403127a(23) =3403127a(24) =19628064a(25) =19628064a(26) =114700315a(27) =114700315a(28) =676207628a(29) =676207628