Number of palindromic structures using exactly four different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =1a(7) =1a(8) =10a(9) =10a(10) =65a(11) =65a(12) =350a(13) =350a(14) =1701a(15) =1701a(16) =7770a(17) =7770a(18) =34105a(19) =34105a(20) =145750a(21) =145750a(22) =611501a(23) =611501a(24) =2532530a(25) =2532530a(26) =10391745a(27) =10391745a(28) =42355950a(29) =42355950