A060897 Number of walks of length n on square lattice, starting at origin, staying in first and third quadrants.

1, 4, 12, 44, 144, 528, 1808, 6676, 23536, 87568, 315136, 1180680, 4314560, 16263896, 60138816, 227899484, 850600944, 3238194560, 12177384544, 46542879384, 176110444736, 675431779856, 2568878867200, 9882068082112, 37747540858240, 145593279888736, 558190182662144

Offset: 0

Author

David W. Wilson, May 05 2001

Keywords

• nonn

Comments

Is there a formula analogous to the (conjectured) formula for A060900?

Could be broken into the number of walks that are constrained to a quadrant and the number that cross the origin. (I.e., 2*A005566(n) + 2*A005566(n-2)*A005568(1) + 2*A005566(n-4)*A005568(2) + ... + All terms that cross the origin twice + three times + ... + Cross floor(n/2) times.) - Benjamin Phillabaum, Mar 13 2011

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..750 (first 251 terms from Sean A. Irvine)

Crossrefs

Cf. A005566, A005568, A001700, A060898, A060899, A060900.

Programs

PARI

\\ here B is A005566 and C is aerated A005568 as g.f.'s.
B(n)={sum(n=0, n, x^n*binomial(n, n\2)*binomial(n+1, (n+1)\2), O(x*x^n))}
C(n)={sum(n=0, (n+1)\2, x^(2*n)*binomial(2*n,n)*binomial(2*n+2,n+1)/((n+1)*(n+2)), O(x*x^n))}
seq(n) = {Vec( 1 + 2*(B(n)-1)/(2-C(n)) )} \\ Andrew Howroyd, Jan 05 2023

Formula

G.f.: 1 + 2*(B(x)-1)/(2 - C(x^2)) where B(x) is the g.f. of A005566 and C(x) is the g.f. of A005568. - Andrew Howroyd, Jan 05 2023