

A182947


Number of complete sphere stacks on a triangular base with side length n.


1




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 \ / \
11 11
/ \ 3 / \ / \ 2 / \
/ 2 \ / 2 \ / \ / \
111 111


CROSSREFS

Cf. A182948 = spheres on a rhombic base.


KEYWORD

nonn,more


AUTHOR



STATUS

approved



