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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =3a(6) =7a(7) =44a(8) =90a(9) =375a(10) =715a(11) =2487a(12) =4550a(13) =14350a(14) =25515a(15) =75724a(16) =132090a(17) =376779a(18) =647995a(19) =1798175a(20) =3060736a(21) =8329046a(22) =14064523a(23) =37725791a(24) =63313250a(25) =168011025a(26) =280576935a(27) =738453366a(28) =1228322550a(29) =3212296488