Number of diagonal polyominoes with n cells.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =15a(5) =54a(6) =212a(7) =908a(8) =4011a(9) =18260a(10) =84320a(11) =394462a(12) =1860872a(13) =8843896a(14) =42275308a(15) =203113670a(16) =980101070a(17) =4747504560a(18) =23074132601