OFFSET
0,3
COMMENTS
Also, the number of (w,x,y,z) with all terms in {1,...,n} and H(w,x,y)=H(w,x,y,z) where H denotes harmonic mean. For a guide to related sequences, see A211795.
EXAMPLE
a(4) counts these: (1,1,1,1), (2,1,4,4), (2,2,2,2), (2,4,1,4), (2,4,4,1), (3,2,4,4), (3,3,3,3), (3,4,2,4), (3,4,4,2), (4,4,4,4); e.g., (3,2,4,4) is included because it satisfies 3/w=1/x+1/y+1/z.
MATHEMATICA
t = Compile[{{n, _Integer}}, Module[{s = 0},
(Do[If[w*(y*z + z*x + x*y) == 3 x*y*z, s = s + 1],
{w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]];
Map[t[#] &, Range[0, 60]] (* A212103 *)
(* Peter J. C. Moses, Apr 13 2012 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Clark Kimberling, May 03 2012
STATUS
approved