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A182947 Number of complete sphere stacks on a triangular base with side length n. 1

%I #16 Apr 20 2021 02:41:43

%S 1,1,2,9,76,1134,33464,1951187

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

%C 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.

%C a(n) is the number of distinct complete stacks that can be built.

%e For n=3 there are two complete stacks, so a(3)=2.

%e .

%e 3 layers, 10 spheres 2 layers, 7 spheres

%e .

%e 1 1

%e / \ / \

%e / 2 \ / \

%e 1-----1 1-----1

%e / \ 3 / \ / \ 2 / \

%e / 2 \ / 2 \ / \ / \

%e 1-----1-----1 1-----1-----1

%Y Cf. A182948 = spheres on a rhombic base.

%K nonn,more

%O 1,3

%A _David Scambler_, Dec 14 2010

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Last modified April 15 17:01 EDT 2024. Contains 371693 sequences. (Running on oeis4.)