A085973 Number of ways a loop can cross two parallel roads 2n times.

3, 2, 5, 22, 123, 800, 5754, 44514, 363893, 3106288, 27457050, 249768040, 2327398572, 22135606604, 214270565106, 2106151496858, 20982672402385, 211545853142240, 2155553788108702, 22174250217880984, 230075164780356214

Offset

0,1

Comments

There is no obligation to cross the lower road (cf. A077054).

Links

Table of n, a(n) for n=0..20.

Formula

a(n) = A077054(n) + A005315(n) for n >= 1. - Andrew Howroyd, Nov 26 2015

Mathematica

A005315 = Cases[Import["https://oeis.org/A005315/b005315.txt", "Table"], {_, _}][[All, 2]];
A005316 = Cases[Import["https://oeis.org/A005316/b005316.txt", "Table"], {_, _}][[All, 2]];
a[n_] := If[n == 0, 3, A005315[[n + 1]] + A005316[[2 n + 1]]];
a /@ Range[0, 20] (* Jean-François Alcover, Sep 08 2019 *)

Crossrefs

Cf. A077054, A086031.

Sequence in context: A269155 A103938 A304534 * A302854 A248243 A005265

Adjacent sequences: A085970 A085971 A085972 * A085974 A085975 A085976

Keyword

nonn

Author

N. J. A. Sloane and Jon Wild, Aug 25 2003

Extensions

a(13)-a(20) from Andrew Howroyd, Nov 26 2015

Status

approved