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A182947 Number of complete sphere stacks on a triangular base with side length n. 1
1, 1, 2, 9, 76, 1134, 33464, 1951187 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Start with a layer of spheres closely packed in an equilateral triangle of side n >= 1. Add spheres by resting them in any of the hollows between three touching spheres in the layer below. Continue until no more sites are available.
a(n) is the number of distinct complete stacks that can be built.
LINKS
EXAMPLE
For n=3 there are two complete stacks, so a(3)=2.
.
3 layers, 10 spheres 2 layers, 7 spheres
.
1 1
/ \ / \
/ 2 \ / \
1-----1 1-----1
/ \ 3 / \ / \ 2 / \
/ 2 \ / 2 \ / \ / \
1-----1-----1 1-----1-----1
CROSSREFS
Cf. A182948 = spheres on a rhombic base.
Sequence in context: A058164 A006059 A277406 * A006041 A192551 A121131
KEYWORD
nonn,more
AUTHOR
David Scambler, Dec 14 2010
STATUS
approved

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Last modified February 24 10:53 EST 2024. Contains 370295 sequences. (Running on oeis4.)