Number of palindromic structures using exactly five different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =1a(9) =1a(10) =15a(11) =15a(12) =140a(13) =140a(14) =1050a(15) =1050a(16) =6951a(17) =6951a(18) =42525a(19) =42525a(20) =246730a(21) =246730a(22) =1379400a(23) =1379400a(24) =7508501a(25) =7508501a(26) =40075035a(27) =40075035a(28) =210766920a(29) =210766920