Number of n-step closed paths on hexagonal lattice.

Terms

a(0) =1a(1) =0a(2) =6a(3) =12a(4) =90a(5) =360a(6) =2040a(7) =10080a(8) =54810a(9) =290640a(10) =1588356a(11) =8676360a(12) =47977776a(13) =266378112a(14) =1488801600a(15) =8355739392a(16) =47104393050a(17) =266482019232