OFFSET
1,1
COMMENTS
10^11 < a(4) <= 8581256320000.
The numbers equal to the sum of their proper divisors which are a multiple of 3 are the perfect numbers (A000396) multiplied by 3.
EXAMPLE
The proper divisors of 440 which are congruent to 1 mod 3 are 1, 4, 10, 22, 40, 55, 88, and 220. Since their sum is 440, 440 is a term.
MATHEMATICA
Select[Range[5000], # == Plus @@ Select[Most@ Divisors@#, Mod[#, 3] == 1 &] &]
PROG
(PARI) is(n)=sumdiv(n, d, if(d%3==1, d, 0))==if(n%3==1, 2*n, n) \\ Charles R Greathouse IV, May 09 2016
CROSSREFS
KEYWORD
nonn,bref,more,hard
AUTHOR
Giovanni Resta, May 05 2016
STATUS
approved