Sum of the lengths of the free polyominoes with n cells.

Terms

a(0) =1a(1) =2a(2) =5a(3) =15a(4) =41a(5) =139a(6) =474a(7) =1773a(8) =6686a(9) =26043a(10) =101814a(11) =402297a(12) =1593124a(13) =6332329a(14) =25200575a(15) =100462874a(16) =400908688a(17) =1601541747