OFFSET
0,3
COMMENTS
This is the number of walks from (0,0) to (n,n) where unit horizontal (+1,0), vertical (0,+1), and diagonal (+1,+1) steps are permitted but a horizontal step cannot be followed by a vertical step, nor vice versa.
a(n) is also the number of walks from (0,0) to (n,n) with steps that increment one or two coordinates and having the property that no two consecutive steps are orthogonal. - Lee A. Newberg, Dec 04 2009
a(n) is also the number of standard sequence alignments of two strings of length n, counting only those alignments with the property that, for every pair of consecutive alignment columns, there is at least one sequence that contributes a non-gap to both columns. That is, a(n) counts only those standard alignments with a column order that can be unambiguously reconstructed from the knowledge of all pairings, where a pairing is, e.g., that some i-th position of the first string is in the same column as some j-th position of the second string. - Lee A. Newberg, Dec 11 2009
First differences of A108626: a(n) = A108626(n) - A108626(n-1) for n>=1. - Thomas Baruchel, Nov 08 2014
The number of walls of height one in all bargraphs of semiperimeter n>=2. A wall is a maximal sequence of adjacent up steps. - Arnold Knopfmacher, Nov 04 2016
Main diagonal of Table 2 in Covington. - Peter Bala, Jan 27 2018
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
A. Blecher, C. Brennan and A. Knopfmacher, Walls in bargraphs, preprint.
Denis Bouyssou, Thierry Marchant, and Marc Pirlot, About maximal antichains in a product of two chains:A catch-all note, arXiv:2410.16243 [math.CO], 2024. See pp. 1, 3, 16-18.
Denis Bouyssou, Thierry Marchant, and Marc Pirlot, ELECTRE TRI-nB, pseudo-disjunctive: axiomatic and combinatorial results, arXiv:2410.18443 [cs.DM], 2024. See p. 14.
M. A. Covington, The Number of Distinct Alignments of Two Strings, Journal of Quantitative Linguistics, Vol.11 (2004), Issue3, pp. 173-182
FORMULA
a(n) = ((4*n-3)*a(n-1)-(2*n-5)*a(n-2)+a(n-3)-(n-3)*a(n-4))/n. - Alois P. Heinz, Jan 22 2013
G.f.: sqrt((1-x)/(1-3*x-x^2-x^3)). - Mark van Hoeij, May 10 2013
G.f.: Sum_{n>=0} (2*n)!/n!^2 * x^(2*n) / (1-2*x)^(3*n+1). - Paul D. Hanna, Sep 21 2013
G.f.: Sum_{n>=0} x^n/(1-x)^n * Sum_{k=0..n} C(n,k)^2 * x^k. - Paul D. Hanna, Nov 08 2014
EXAMPLE
For n = 3, the 9 alignments are:
ABC A-BC ABC- -ABC -ABC --ABC ABC- AB-C ABC--
DEF DEF- D-EF DEF- DE-F DEF-- -DEF -DEF --DEF
MAPLE
a:= proc(n) option remember; `if`(n<4, [1, 1, 3, 9][n+1],
((4*n-3)*a(n-1) -(2*n-5)*a(n-2) +a(n-3) -(n-3)*a(n-4))/n)
end:
seq(a(n), n=0..30); # Alois P. Heinz, Jan 22 2013
MATHEMATICA
CoefficientList[Series[Sqrt[(1 - x) / (1 - 3 x - x^2 - x^3)], {x, 0, 40}], x] (* Vincenzo Librandi, Nov 09 2014 *)
PROG
(PARI) x='x+O('x^66); Vec(sqrt((1-x)/(1-3*x-x^2-x^3))) \\ Joerg Arndt, May 11 2013
(PARI) {a(n)=polcoeff(sum(m=0, n, (2*m)!/m!^2 * x^(2*m) / (1-x+x*O(x^n))^(3*m+1)), n)} \\ Paul D. Hanna, Sep 21 2013
(PARI) {a(n)=polcoeff( sum(m=0, n, x^m * sum(k=0, m, binomial(m, k)^2 * x^k) / (1-x +x*O(x^n))^m) , n)}
for(n=0, 30, print1(a(n), ", ")) \\ Paul D. Hanna, Nov 08 2014
(PARI) a(n)=sum(k=0, n, sum(i=0, k, binomial(n-k, i)^2*binomial(n-i, k-i)))-sum(k=0, n-1, sum(i=0, k, binomial(n-k-1, i)^2*binomial(n-i-1, k-i))) \\ Thomas Baruchel, Nov 09 2014
CROSSREFS
See A171158 for the number of such walks in three dimensions. - Lee A. Newberg, Dec 04 2009
See A171563 for the number of such walks in four dimensions. - Lee A. Newberg, Dec 11 2009
Cf. A108626.
Main diagonal of A180091.
KEYWORD
nonn,walk
AUTHOR
Lee A. Newberg, Dec 04 2009
EXTENSIONS
Extended beyond a(19) by Alois P. Heinz, Jan 22 2013
STATUS
approved