Number of n X n 0..6 matrices with each 2X2 subblock idempotent.

Terms

a(0) =7a(1) =28a(2) =162a(3) =268a(4) =596a(5) =1118a(6) =2166a(7) =4038a(8) =7488a(9) =13686a(10) =24818a(11) =44640a(12) =79796a(13) =141848a(14) =250966a(15) =442168a(16) =776152a(17) =1357850a(18) =2368298a(19) =4119186