34984domain: NAppears in sequencesNumber of planar n X n X n binary triangular grids symmetric under 120-degree rotation with no more than 1 one in any 4 X 4 X 4 subtriangle.at n=20A153902Number of idempotent n X n 0..2 matrices of rank n-1.at n=7A224327T(n,k)=Number of idempotent n X n 0..k matrices of rank n-1.at n=43A224333