Palindromes using exactly four different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =24a(7) =24a(8) =240a(9) =240a(10) =1560a(11) =1560a(12) =8400a(13) =8400a(14) =40824a(15) =40824a(16) =186480a(17) =186480a(18) =818520a(19) =818520a(20) =3498000a(21) =3498000a(22) =14676024a(23) =14676024a(24) =60780720a(25) =60780720a(26) =249401880a(27) =249401880a(28) =1016542800a(29) =1016542800