Number of idempotent 3X3 -n..n matrices of rank 2.

Terms

a(0) =75a(1) =399a(2) =927a(3) =1815a(4) =2751a(5) =4839a(6) =6231a(7) =8403a(8) =10683a(9) =14283a(10) =16491a(11) =21747a(12) =24483a(13) =29499a(14) =35067a(15) =40371a(16) =43851a(17) =52275a(18) =56307a(19) =65187a(20) =73203a(21) =81387a(22) =86091a(23) =98787a(24) =105987a(25) =115539a(26) =124179a(27) =136947a(28) =143139a(29) =163899