OFFSET
1,1
COMMENTS
The numbers prime(n)^3 are in the sequence.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
sopfr(n) = 3*gpf(n), where gpf = A006530. - Charles R Greathouse IV, May 29 2012
EXAMPLE
150 is in the sequence because 150 = 2*3*5^2 => sum of prime divisors = 2+3 + 5*2 = 15 = 3*5 where 5 is the greatest prime divisor.
MAPLE
with(numtheory):A:= proc(n) local e, j; e := ifactors(n)[2]: add (e[j][1]*e[j][2], j=1..nops(e)) end: for m from 2 to 20000 do: x:=factorset(m):n1:=nops(x):if A(m)=3*x[n1] then printf(`%d, `, m):else fi:od:
MATHEMATICA
spfQ[n_]:=Module[{f=FactorInteger[n]}, Total[Flatten[Table[#[[1]], #[[2]]]&/@ f]]==3*f[[-1, 1]]]; Select[Range[16000], spfQ] (* Harvey P. Dale, Jul 26 2016 *)
PROG
(PARI) is(n)=my(f=factor(n), k=#f[, 1]); k && sum(i=1, k, f[i, 1]*f[i, 2]) == 3*f[k, 1] \\ Charles R Greathouse IV, May 29 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, May 29 2012
STATUS
approved