OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..100
FORMULA
a(n) ~ c * 26^n / n^(3/2), where c = 0.1102253437... . - Vaclav Kotesovec, Sep 07 2014
MAPLE
b:= proc(n, p) option remember; `if`(p[3]>n, 0, `if`(n=0, 1,
add(add(add(`if`(i=0 and j=0 and k=0, 0, b(n-1, sort(map(abs,
p+[i, j, k])))), i=-1..1), j=-1..1), k=-1..1)))
end:
a:= n-> b(n, [0$2, 1]):
seq (a(n), n=0..25); # Alois P. Heinz, Nov 28 2012
MATHEMATICA
b[n_, p_] := b[n, p] = If[p[[3]]>n, 0, If[n==0, 1, Sum[Sum[Sum[If[i==0 && j==0 && k==0, 0, b[n-1, Sort[Map[Abs, p + {i, j, k}]]]], {i, -1, 1}], {j, -1, 1}], {k, -1, 1}]]];
a[n_] := b[n, {0, 0, 1}];
a /@ Range[0, 25] (* Jean-François Alcover, Dec 20 2020, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Jon Perry, Nov 24 2012
EXTENSIONS
More terms from Alois P. Heinz, Nov 28 2012
STATUS
approved