15690441231domain: NAppears in sequencesa(n) = 3^n*(3^n-1)/2.at n=11A026121Number of reversible strings with n beads of 3 colors. If more than 1 bead, not palindromic.at n=21A032086Expansion of x^2/((3*x-1)*(3*x^2-1)).at n=23A167993