Palindromes using exactly six different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =0a(10) =720a(11) =720a(12) =15120a(13) =15120a(14) =191520a(15) =191520a(16) =1905120a(17) =1905120a(18) =16435440a(19) =16435440a(20) =129230640a(21) =129230640a(22) =953029440a(23) =953029440a(24) =6711344640a(25) =6711344640a(26) =45674188560