Number of non-attacking bishop positions on a 3 X n chessboard.

Terms

a(0) =1a(1) =8a(2) =25a(3) =70a(4) =225a(5) =748a(6) =2401a(7) =7668a(8) =24649a(9) =79344a(10) =255025a(11) =819494a(12) =2634129a(13) =8467464a(14) =27217089a(15) =87483296a(16) =281199361a(17) =903867144a(18) =2905317801a(19) =9338615022a(20) =30017295025a(21) =96485195716a(22) =310134268609a(23) =996870677460