A026087 a(n) = number of (s(0), s(1), ..., s(n)) such that every s(i) is an integer, s(0) = 0, |s(i) - s(i-1)| = 1 for i = 1,2,3; |s(i) - s(i-1)| <= 1 for i >= 4, s(n) = 4. Also a(n) = T(n,n-4), where T is the array defined in A026082.

1, 2, 9, 28, 93, 294, 925, 2872, 8856, 27136, 82764, 251472, 761774, 2301924, 6941898, 20899680, 62834397, 188690634, 566081421, 1696873148, 5082959517, 15216909686, 45532045749, 136182428520, 407160436435, 1216953379486, 3636353333187

Offset: 4

Author

Clark Kimberling

Keywords

• nonn

Crossrefs

First differences of A026071.

Formula

Conjecture: -(n-4)*(n+4)*a(n) +(4*n^2-11*n-27)*a(n-1) +(-2*n^2+29*n-24)*a(n-2) -(4*n+5)*(n-5)*a(n-3) +3*(n-5)*(n-6)*a(n-4)=0. - R. J. Mathar, Jun 23 2013