Number of unrestricted planar Langford sequences.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =0a(5) =0a(6) =6a(7) =24a(8) =0a(9) =0a(10) =139a(11) =289a(12) =0a(13) =0a(14) =2414a(15) =4455a(16) =0a(17) =0a(18) =33222a(19) =63700a(20) =0a(21) =0a(22) =437489a(23) =794953a(24) =0a(25) =0