
COMMENTS

In the notation of Nemirovsky et al. (1992), a(n), the nth term of the current sequence, is equal to p_{n,m}^{(l)} with m = 0 and l = 3.  Petros Hadjicostas, Jan 02 2019
This counts nonselfintersecting paths of length n on the cubic lattice, start and end points distinguished, planar paths not counted, rotations and reflections of a path not counted as distinct from that path. No points repeated, no adjacent points allowed unless consecutive in path.  Bert Dobbelaere, Jan 03 2019


EXAMPLE

From Bert Dobbelaere, Jan 03 2019: (Start)
Using strings to represent a path with characters X,Y,Z for steps in positive directions and x,y,z for steps in negative directions along the respective axes, the following enumerations correspond to the first nonzero terms:
a(3) = 1: { XYZ }
a(4) = 7: { XXYZ, XYXZ, XYYZ, XYZX, XYZx, XYZY, XYZZ }
a(5) = 36: {
XXXYZ, XXYXZ, XXYYZ, XXYZX, XXYZx, XXYZY, XXYZZ, XYXXZ, XYXYZ,
XYXZX, XYXZY, XYXZy, XYXZZ, XYYXZ, XYYxZ, XYYYZ, XYYZX, XYYZx,
XYYZY, XYYZZ, XYZXX, XYZXY, XYZXy, XYZXZ, XYZxx, XYZxY, XYZxZ,
XYZYX, XYZYx, XYZYY, XYZYZ, XYZZX, XYZZx, XYZZY, XYZZy, XYZZZ }
Symmetries are avoided by imposing the following restrictions: all patterns start with 'X'. First occurrence of 'Y' comes before the first occurrence of 'Z' (presence mandatory). First occurrence of steps in negative directions (presence optional) comes after the first occurrence of the corresponding steps in positive directions.
(End)
