273752domain: NAppears in sequencesNumber of planar n X n X n binary triangular grids symmetric under 120 degree rotation with no more than 2 ones in any 2 X 2 X 2 subtriangle.at n=11A153908a(n) = Sum_{k=0..n} binomial(n,k) * binomial(4*n+2*k,k).at n=5A387427