T(n,k)=Number of idempotent n X n -k..k matrices of rank 2.

Terms

a(0) =0a(1) =0a(2) =1a(3) =0a(4) =1a(5) =75a(6) =0a(7) =1a(8) =399a(9) =4470a(10) =0a(11) =1a(12) =927a(13) =90390a(14) =224810a(15) =0a(16) =1a(17) =1815a(18) =424374