OFFSET
1,1
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..361
Trygve Breiteig, When is the product of two oblong numbers another oblong?, Math. Mag. 73 (2000), 120-129.
EXAMPLE
11310 = 5 * 2262; that is, pen(87) = pen(2) * pen(39).
MATHEMATICA
PentagonalQ[n_] := IntegerQ[(1 + Sqrt[1 + 24*n])/6]; PenIndex[n_] := Floor[(1 + Sqrt[1 + 24*n])/6]; lim = 10^10; nMax = PenIndex[lim/5]; pen = Table[n (3 n - 1)/2, {n, 2, nMax}]; Union[Reap[Do[num = pen[[i]]*pen[[j]]; If[PentagonalQ[num], Sow[num]], {i, PenIndex[Sqrt[lim]]}, {j, i, PenIndex[lim/pen[[i]]] - 1}]][[2, 1]]]
CROSSREFS
KEYWORD
nonn
AUTHOR
T. D. Noe, Apr 07 2011
STATUS
approved