OFFSET
1,1
COMMENTS
That is, pen(k) = k*(3k-1)/2.
EXAMPLE
For n = 2, pen(n) = 5 and the first k is 39 because pen(39) = 2262 and 5*2262 = 11310 which is the 87th pentagonal number.
MATHEMATICA
kMax = 10^7; PentagonalQ[n_] := IntegerQ[(1 + Sqrt[1 + 24*n])/6]; Table[t = n*(3*n - 1)/2; k = 2; While[t2 = k*(3*k - 1)/2; k < kMax && ! PentagonalQ[t*t2], k++]; If[k == kMax, 0, k], {n, 15}]
CROSSREFS
KEYWORD
nonn
AUTHOR
T. D. Noe, Jun 07 2012
EXTENSIONS
a(25) corrected and a(28)-a(42) from Donovan Johnson, Feb 08 2013
STATUS
approved