OFFSET
1,2
COMMENTS
Start with a layer of spheres closely packed in a rhombus (diamond) 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.
EXAMPLE
For n=2 there are two complete stacks so a(2)=2.
.
1-----------1 1-----------1
\ / \ \ / \
\ 2 / \ \ / \
\ / \ or \ / \
\ / \ \ / 2 \
\ / \ \ / \
1-----------1 1-----------1
CROSSREFS
KEYWORD
nonn,more
AUTHOR
David Scambler, Dec 14 2010
STATUS
approved