Number of idempotent 3 X 3 0..n matrices of rank 2.

Terms

a(0) =21a(1) =51a(2) =93a(3) =147a(4) =213a(5) =291a(6) =381a(7) =483a(8) =597a(9) =723a(10) =861a(11) =1011a(12) =1173a(13) =1347a(14) =1533a(15) =1731a(16) =1941a(17) =2163a(18) =2397a(19) =2643a(20) =2901a(21) =3171a(22) =3453a(23) =3747a(24) =4053a(25) =4371a(26) =4701a(27) =5043a(28) =5397a(29) =5763