OFFSET
1,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..5000
EXAMPLE
a(3) = 35 because sopfr(35) = sum of prime factors of 35 = 5 + 7 = 12, and 12 is the 3rd pentagonal number.
MATHEMATICA
pentagonalQ[n_] := IntegerQ[(1 + Sqrt[1 + 24*n])/6]; pfs[n_] := Module[{p, e}, {p, e} = Transpose[FactorInteger[n]]; Dot[p, e]]; Select[Range[1500], pentagonalQ[pfs[#]] &] (* T. D. Noe, May 31 2012 *)
PROG
(PARI) sopfr(n) = my(f=factor(n)); sum(k=1, matsize(f)[1], f[k, 1]*f[k, 2]); \\ A001414
isok(n) = ispolygonal(sopfr(n), 5); \\ Michel Marcus, May 02 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, May 31 2012
EXTENSIONS
Corrected by T. D. Noe, May 31 2012
STATUS
approved