OFFSET
1,1
COMMENTS
4*a(n)^2 are the solutions to A048272(x) = -Sum_{d|x} (-1)^d = -9 - Benoit Cloitre, Apr 14 2002
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
A113901(a(n)) = 4. - Reinhard Zumkeller, Mar 13 2011
MATHEMATICA
f[n_] := DivisorSum[n, PrimeNu[#] &]; Select[Range[201], f[#] == 4 &] (* Amiram Eldar, Jul 25 2020 *)
PROG
(PARI) for(n=1, 100, if(sumdiv(n, d, omega(d))==4, print1(n, ", ")))
(PARI) is(n)=my(f=factor(n)[, 2]~); f==[1, 1] || f==[4] \\ Charles R Greathouse IV, Oct 15 2015
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Apr 06 2002
STATUS
approved