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

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =5a(5) =21a(6) =42a(7) =122a(8) =225a(9) =570a(10) =990a(11) =2321a(12) =3913a(13) =8827a(14) =14480a(15) =31802a(16) =51425a(17) =110928a(18) =177270a(19) =377722a(20) =598437a(21) =1264307a(22) =1990098a(23) =4175201a(24) =6540605a(25) =13653822a(26) =21301740a(27) =44288071a(28) =68877929a(29) =142736091