0,3

Form a graph from a triangle and its midpoint triangle. A093121 counts closed walks of length n at a vertex of the midpoint triangle in this configuration.

G.f.: (1-x-3x^2)/((1+x-x^2)(1-2x-4x^2)); a(n)=A078008(n)*A000045(n+1); a(n)=(2^n/3+2(-1)^n/3)(((1+sqrt(5))/2)^(n+1)/sqrt(5)-((1-sqrt(5))/2)^(n+1)/sqrt(5)).

Sequence in context: A056221 A046849 A090530 this_sequence A136351 A075590 A088255

Adjacent sequences: A093118 A093119 A093120 this_sequence A093122 A093123 A093124

easy,nonn

Paul Barry (pbarry(AT)wit.ie), Mar 22 2004