OFFSET
0,4
COMMENTS
a(n) is also the number of different ways to fold down a folding yardstick with n sticks (stick length is one) to form an equilateral triangle with side length one. E.g., mark the vertices of the triangle as 1, 2 and 3. Having a yardstick with n = 3 sticks there are two ways of folding to form an equilateral triangle: 1-2-3-1 and 1-3-2-1. For n = 4 there are six ways of folding: 1-2-3-1-2 or 3; 1-3-2-1-2 or 3; 1-2-1-3-2 and 1-3-1-2-3. - Frank Hollstein, Apr 23 2019
LINKS
Index entries for linear recurrences with constant coefficients, signature (3,0,-6,4).
FORMULA
a(n) = 2^n - A027383(n).
G.f.: 2*x^2/((1-x)*(1-2*x)(1-2*x^2)).
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved