Number of periodic palindromes using a maximum of six different symbols.

Terms

a(0) =6a(1) =21a(2) =36a(3) =126a(4) =216a(5) =756a(6) =1296a(7) =4536a(8) =7776a(9) =27216a(10) =46656a(11) =163296a(12) =279936a(13) =979776a(14) =1679616a(15) =5878656a(16) =10077696a(17) =35271936a(18) =60466176a(19) =211631616a(20) =362797056a(21) =1269789696a(22) =2176782336a(23) =7618738176a(24) =13060694016a(25) =45712429056a(26) =78364164096a(27) =274274574336a(28) =470184984576