Number of n-step anisotropic spirals on cubic lattice.

Terms

a(0) =1a(1) =6a(2) =24a(3) =90a(4) =324a(5) =1166a(6) =4138a(7) =14730a(8) =51992a(9) =183898a(10) =646980a(11) =2279702a(12) =8002976a(13) =28127418a(14) =98585096a(15) =345848306a(16) =1210704274a(17) =4241348770a(18) =14833284544a(19) =51907058582a(20) =181392476966a(21) =634197818374