Number of n-step spirals on cubic lattice.

Terms

a(0) =1a(1) =6a(2) =18a(3) =54a(4) =150a(5) =426a(6) =1158a(7) =3204a(8) =8682a(9) =23724a(10) =64194a(11) =174378a(12) =470856a(13) =1274430a(14) =3434826a(15) =9272346a(16) =24953004a(17) =67230288a(18) =180705126a(19) =486152604a(20) =1305430884a(21) =3507947838a(22) =9412114986a(23) =25268587338a(24) =67752451146a(25) =181754458194a(26) =487060621596