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 A272716 Numbers equal to the sum of their proper divisors d such that d mod 3 = 1. 2
 440, 28109312, 79228362752 (list; graph; refs; listen; history; text; internal format)
 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. LINKS 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 Cf. A272715, A000396. Sequence in context: A234202 A061626 A187239 * A124170 A252193 A207650 Adjacent sequences:  A272713 A272714 A272715 * A272717 A272718 A272719 KEYWORD nonn,bref,more,hard AUTHOR Giovanni Resta, May 05 2016 STATUS approved

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Last modified February 17 12:14 EST 2020. Contains 331996 sequences. (Running on oeis4.)