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This sequence gives the number of self-avoiding walks on a 2-dimensional square lattice where the walk starts with a step length of 1 which then increments by 1 after each step up until the step length is n.

The first time a collision with a previous step can occur is for n = 6. This can occur in three different ways. For example a walk with steps of length 1,2 and 3 to the right, a step of length 4 upward, then a step of length 5 to the left. A step of length 6 downward would now result in a collision. Requiring six steps before a collision is in contrast to the standard 2D square lattice SAW of A001411 where a collision can occur on the fourth step.

Note that this sequence agrees with a SAW on the diamond lattice, A001394, for the first 7 terms, even though the seventh term here has some walks removed due to the above collision.

Trails are directed and pass through each (undirected) edge at most once in either of the two directions.

Let m >= 1. The sequence of hypergraph Catalan numbers {C_m(n): n >= 0} is defined in terms of counting walks on trees, weighted by the orders of their automorphism groups. When m = 1 we get the sequence of Catalan numbers A000108. The present sequence is the case m = 2.

Let T(n) be the set of unlabeled trees on n vertices (see A000055). Let T be a tree in T(n+1), and let v be a vertex of T. Then an a(m,T)-tour beginning at v is a walk that begins and ends at v and traverses each edge of T exactly 2*m times. We denote by a(m,T)(v) the number of a(m,T)-tours beginning at v.

The hypergraph Catalan numbers C_m(n) are defined by C_m(n) = Sum_{trees T in T(n+1)} Sum_{vertices v in T} a(m,T)(v)/|Aut(T)|, where Aut(T) denotes the automorphism group of the tree T.

Gunnells gives several combinatorial interpretations of the hypergraph Catalan numbers, a method to compute their generating functions to arbitrary precision and some conjectural asymptotics.

Let m >= 1. The sequence of hypergraph Catalan numbers {C_m(n): n >= 0} is defined in terms of counting walks on trees, weighted by the orders of their automorphism groups. See Gunnells. When m = 1 we get the sequence of Catalan numbers A000108. The present sequence is the case m = 7.

Gunnells gives several combinatorial interpretations of the hypergraph Catalan numbers, a method to compute their generating functions to arbitrary precision and some conjectural asymptotics.

A "polygon" is a self-avoiding walk from (0,0) to (0,0).

The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.

Number of 2 X n binary matrices avoiding simultaneously the right-angled numbered polyomino patterns (ranpp) (00;1) and (01;1). An occurrence of a ranpp (xy;z) in a matrix A=(a(i,j)) is a triple (a(i1,j1), a(i1,j2), a(i2,j1)) where i1 < i2, j1 < j2 and these elements are in same relative order as those in the triple (x,y,z). - Sergey Kitaev, Nov 11 2004

Original name: Number of 2n-step polygons on the cubic lattice. "Polygons" suggests translation invariance, and/or independence of the starting point; those are counted in A001409. Here, two paths which start and end at the origin are counted as distinct if they have a different sequence of steps, even if their set of edges is the same modulo translations.

a(n) is the number of 2n-step closed self-avoiding paths on the cubic lattice. - Bert Dobbelaere, Jan 04 2019

Used as the denominator for the mean square displacement of all different self-avoiding n-step walks in A078797. - Hugo Pfoertner, Dec 09 2002

Number of ways a toy snake with n segments can be bent without flipping the snake upside down. Each segment must be perpendicular or parallel with each adjacent segment. A "slither" is a way of writing down the configuration of a snake; starting from the tail, write down which direction the next segment is pointing (R for right, S for straight, L for left). E.g., a snake with 10 segments may have the valid slither RLRRLLRRL, but not RSRRSSLSL.

From Michel Marcus and Petros Hadjicostas, Jul 16 2020: (Start)

a(n) is the number of subdivisions of a 2 x n grid as defined in Robeva and Sun (2020). We have a(n) = A059576(n-1, n-1) for n >= 1 privided the latter is viewed as a square array (rather than a triangle).

In general, A059576(m-1, n-1) is the number of subdivisions of a 2-row grid with m points at the top row and n points at the bottom. (End)

The title condition is unclear: the path (0,0) -> (0,n) -> (n,n-1) -> (n,n) arguably meets the title condition but is not allowed, because steps with negative slope are proscribed. Steps must move east (slope 0) or have finite positive slope or move north (infinite slope). On the other hand, for lattice paths subject only to the condition that each successive point on the path is closer to the terminal point than its predecessor, see the question "Why are the numbers counting "ever-closer" lattice paths so round?" on the mathoverflow website. - David Callan, Nov 21 2021

Let P(A) be the power set of an n-element set A. Then a(n) = the total number of ways to add 1 or more elements of A to each element x of P(A) where the elements to add are not elements of x and order of addition is important. - Ross La Haye, Nov 19 2007