A026289 a(n) = number of (s(0), s(1), ..., s(n)) such that every s(i) is an integer, s(0) = 0, s(1) = 1, s(n) = 3, |s(i) - s(i-1)| <= 1 for i >= 2, |s(2) - s(1)| = 1, |s(3) - s(2)| = 1 if s(2) = 1. Also a(n) = T(n,n-3), where T is the array in A026268.

1, 3, 9, 27, 79, 229, 659, 1889, 5402, 15430, 44054, 125786, 359296, 1026936, 2937444, 8409540, 24097737, 69118635, 198442329, 570286939, 1640469427, 4723363073, 13612376671, 39265012213, 113358893147, 327545797361, 947203621523, 2741308151929, 7939698087777

Offset: 3

Author

Clark Kimberling

Keywords

• nonn

Crossrefs

Pairwise sums of A026124.

Formula

G.f.: 16*z^3*(1-z)*(1-z^2)/[1-z+sqrt(1-2*z-3*z^2)]^4.

Extensions

a(27) corrected and more terms from Sean A. Irvine, Sep 24 2019