Number of even-length columns in all bargraphs having semiperimeter n (n>=2).

Terms

a(0) =0a(1) =1a(2) =4a(3) =13a(4) =44a(5) =149a(6) =498a(7) =1656a(8) =5498a(9) =18236a(10) =60456a(11) =200409a(12) =664464a(13) =2203755a(14) =7311894a(15) =24271290a(16) =80605250a(17) =267821525a(18) =890305418a(19) =2961015981a(20) =9852481830a(21) =32798011430a(22) =109229396466a(23) =363927233758