Number of reversible strings with n beads of 3 colors.

Terms

a(0) =1a(1) =3a(2) =6a(3) =18a(4) =45a(5) =135a(6) =378a(7) =1134a(8) =3321a(9) =9963a(10) =29646a(11) =88938a(12) =266085a(13) =798255a(14) =2392578a(15) =7177734a(16) =21526641a(17) =64579923a(18) =193720086a(19) =581160258a(20) =1743421725a(21) =5230265175a(22) =15690618378a(23) =47071855134a(24) =141215033961a(25) =423645101883