Number of double palindrome structures of length n using a maximum of four different symbols.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =21a(6) =50a(7) =99a(8) =246a(9) =445a(10) =1157a(11) =2047a(12) =5326a(13) =9283a(14) =24453a(15) =41859a(16) =110486a(17) =187851a(18) =494411a(19) =834975a(20) =2190965a(21) =3680541a(22) =9626773a(23) =16101703a(24) =41973942a(25) =69956171a(26) =181823735a(27) =302099557a(28) =783084053a(29) =1297675587