Number of chessboard polyominoes with n squares.

Terms

a(0) =2a(1) =1a(2) =4a(3) =7a(4) =24a(5) =62a(6) =216a(7) =710a(8) =2570a(9) =9215a(10) =34146a(11) =126853a(12) =477182a(13) =1802673a(14) =6853152a(15) =26153758a(16) =100215818a(17) =385226201a(18) =1485248464a(19) =5741275753a(20) =22246121356a(21) =86383454582a(22) =336094015456