OFFSET
0,3
COMMENTS
For a guide to related sequences, see A211795.
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..10000
EXAMPLE
G.f. = x + 2*x^2 + 3*x^3 + 10*x^4 + 11*x^5 + 12*x^6 + 13*x^7 + 26*x^8 + ...
a(4) counts these ten 4-tuples:
(1,1,1,1), (2,2,2,2), (3,3,3,3), (4,4,4,4),
(2,1,2,4), (2,1,4,2), (2,2,1,4), (2,2,4,1),
(2,4,1,2), (2,4,2,1).
MATHEMATICA
t = Compile[{{n, _Integer}}, Module[{s = 0},
(Do[If[w^3 == x*y*z, s = s + 1],
{w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]];
Map[t[#] &, Range[0, 60]] (* A212067 *)
(* Peter J. C. Moses, Apr 13 2012 *)
a[ n_] := Length@FindInstance[ w^3 == x y z && 0 < w <= n && 0 < x <= n && 0 < y <= n && 0 < z <= n, {w, x, y, z}, Integers, 10^9]; (* Michael Somos, Nov 26 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Clark Kimberling, Apr 30 2012
STATUS
approved