# Problem 382: Generating polygons
A polygon is a flat shape consisting of straight line segments that are
joined to form a closed chain or circuit. A polygon consists of at least
three sides and does not self-intersect. A set S of positive numbers is
said to generate a polygon P if: no two sides of P are the same length,
the length of every side of P is in S, and S contains no other value.
For example: The set {3, 4, 5} generates a polygon with sides 3, 4, and
5 (a triangle). The set {6, 9, 11, 24} generates a polygon with sides 6,
9, 11, and 24 (a quadrilateral). The sets {1, 2, 3} and {2, 3, 4, 9} do
not generate any polygon at all. Consider the sequence s, defined as
follows:s1 = 1, s2 = 2, s3 = 3 sn = sn-1 + sn-3 for n > 3. Let Un be
the set {s1, s2, ..., sn}. For example, U10 = {1, 2, 3, 4, 6, 9, 13, 19,
28, 41}. Let f(n) be the number of subsets of Un which generate at least
one polygon. For example, f(5) = 7, f(10) = 501 and f(25) = 18635853.
Find the last 9 digits of f(1018).