Number of n X n 0..3 matrices with each 2 X 2 subblock idempotent.

Terms

a(0) =4a(1) =16a(2) =72a(3) =142a(4) =332a(5) =668a(6) =1356a(7) =2634a(8) =5046a(9) =9480a(10) =17594a(11) =32286a(12) =58730a(13) =106028a(14) =190204a(15) =339322a(16) =602416a(17) =1064888a(18) =1875104a(19) =3290166a(20) =5754586a(21) =10035236a(22) =17452462a(23) =30275242a(24) =52395822