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

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =21a(6) =47a(7) =92a(8) =202a(9) =355a(10) =782a(11) =1332a(12) =2839a(13) =4733a(14) =10103a(15) =16344a(16) =34762a(17) =55761a(18) =117632a(19) =186980a(20) =392790a(21) =619805a(22) =1297727a(23) =2037180a(24) =4248151a(25) =6642921a(26) =13812710a(27) =21522622a(28) =44630687a(29) =69353037