Number of periodic palindromic structures using a maximum of four different symbols.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =5a(5) =5a(6) =13a(7) =15a(8) =40a(9) =51a(10) =133a(11) =187a(12) =479a(13) =715a(14) =1815a(15) =2795a(16) =7036a(17) =11051a(18) =27747a(19) =43947a(20) =110061a(21) =175275a(22) =438683a(23) =700075a(24) =1750959a(25) =2798251a(26) =6997643a(27) =11188907a(28) =27975351a(29) =44747435