Number of (n+1)X(n+1) 0..2 symmetric matrices with every 2X2 subblock having one or two distinct values, and new values 0..2 introduced in lower triangle row major order.

A210855

Number of (n+1)X(n+1) 0..2 symmetric matrices with every 2X2 subblock having one or two distinct values, and new values 0..2 introduced in lower triangle row major order.