A171761 Number of ternary squareful words of length n.

0, 0, 3, 15, 63, 213, 687, 2127, 6483, 19575, 58905, 176943, 531177, 1593981, 4782513, 14348289, 43045923, 129139119, 387419097, 1162259637, 3486782013, 10460350023, 31381055463, 94143173409

Offset: 0

Author

Mats Granvik, Dec 18 2009

Keywords

• nonn

Crossrefs

Cf. A000244, A006156.

Programs

Mathematica

nmax = 23; 3^Range[0, nmax] - Length /@ NestList[ DeleteCases[ Flatten[ Outer[ Append, #, {1, 2, 3}, 1], 1], {_, x__, x__, _}] &, {{}}, nmax] (* Jean-François Alcover, Jun 12 2017, using Vladimir Reshetnikov's code for A006156 *)

Formula

a(n) = A000244(n) - A006156(n).