Number of periodic palindromic structures of length n using exactly four different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =7a(8) =10a(9) =43a(10) =65a(11) =220a(12) =350a(13) =1059a(14) =1701a(15) =4803a(16) =7770a(17) =21105a(18) =34105a(19) =90248a(20) =145750a(21) =379391a(22) =611501a(23) =1573432a(24) =2532530a(25) =6465473a(26) =10391745a(27) =26379908a(28) =42355950a(29) =107091559