Number of primitive (aperiodic) 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) =8376a(14) =40824a(15) =40800a(16) =186480a(17) =186240a(18) =818520a(19) =818280a(20) =3497976a(21) =3496440a(22) =14676024a(23) =14674440a(24) =60780720a(25) =60772320a(26) =249401640a(27) =249393480a(28) =1016542800a(29) =1016501736