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 A063891 Numbers k such that nusigma(usigma(k)) = 2k, where usigma(k) is the sum of unitary divisors of k (A034448) and nusigma(k) is the sum of non-unitary divisors of k (A048146). 0
 1631, 2016, 8928, 11808, 36576, 45360, 1486080, 2359008, 3093552, 37748448, 101350656, 150994656, 2885670144 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(14) > 2*10^11. All the numbers of the form 2^5 * 3^2 * p where p>3 is a  Mersenne prime (A000668) are in the sequence, so a(14) <= 618475290336. - Giovanni Resta, Apr 10 2019 LINKS MATHEMATICA usigma[1] = 1; usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); nusigma[n_] := DivisorSigma[1, n] - usigma[n]; Select[Range[12000], nusigma[usigma[#]] == 2# &] (* Amiram Eldar, Apr 10 2019 *) PROG (PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)); z(n)=sigma(n)-u(n) ; for(n=1, 10^8, if(z(u(n))==2*n, print1(n, ", "))) CROSSREFS Cf. A000203, A034448, A048146, A019284, A063885. Sequence in context: A241493 A328452 A202985 * A254849 A237923 A297659 Adjacent sequences:  A063888 A063889 A063890 * A063892 A063893 A063894 KEYWORD more,nonn AUTHOR Jason Earls, Aug 28 2001 EXTENSIONS More terms from Thomas Baruchel, Oct 22 2003 a(11)-a(13) from Amiram Eldar, Apr 10 2019 STATUS approved

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Last modified May 16 14:39 EDT 2021. Contains 343949 sequences. (Running on oeis4.)