The number of n-cell polyominoes with a line of symmetry parallel to the edges.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =4a(5) =8a(6) =12a(7) =28a(8) =44a(9) =98a(10) =157a(11) =359a(12) =583a(13) =1324a(14) =2183a(15) =4961a(16) =8263a(17) =18729a(18) =31477a(19) =71231a(20) =120605a(21) =272380a(22) =464188a(23) =1046516a(24) =1793505a(25) =4036645a(26) =6952368a(27) =15623258a(28) =27026985a(29) =60645746