Number of n X n 0..5 matrices with each 2X2 subblock idempotent.

Terms

a(0) =6a(1) =24a(2) =128a(3) =222a(4) =500a(5) =956a(6) =1876a(7) =3538a(8) =6622a(9) =12200a(10) =22274a(11) =40302a(12) =72418a(13) =129332a(14) =229780a(15) =406378a(16) =715800a(17) =1256248a(18) =2197512a(19) =3832510a(20) =6665570a(21) =11563364a(22) =20012598