Number of perfect matchings on an n X n L-shaped graph.

Terms

a(0) =0a(1) =2a(2) =4a(3) =12a(4) =30a(5) =80a(6) =208a(7) =546a(8) =1428a(9) =3740a(10) =9790a(11) =25632a(12) =67104a(13) =175682a(14) =459940a(15) =1204140a(16) =3152478a(17) =8253296a(18) =21607408a(19) =56568930a(20) =148099380a(21) =387729212a(22) =1015088254a(23) =2657535552a(24) =6957518400a(25) =18215019650a(26) =47687540548a(27) =124847601996a(28) =326855265438a(29) =855718194320