

A045768


Numbers k such that sigma(k) == 2 (mod k).


26



1, 20, 104, 464, 650, 1952, 130304, 522752, 8382464, 134193152, 549754241024, 8796086730752, 140737463189504
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OFFSET

1,2


COMMENTS

Equivalently, Chowla function of k is congruent to 1 (mod k).
If p=2^i3 is prime, then 2^(i1)*p is a term of the sequence. 650 is in the sequence, but is not of this form.
For 1 < k <= 140737463189504 this sequence has the property that if sigma(k) == 2 (mod k) then sigma(k) == 0 (mod k+1). It is not known if this holds in general, for there might be solutions of sigma(k)=3k+2 or 4k+2 or ... (Comments from Jud McCranie and Dean Hickerson, updated by Jon E. Schoenfield, Sep 25 2021).
k  sigma(k) produces the multiperfect numbers (A007691). It is an open question whether k  sigma(k)  1 iff k is a prime or 1. It is not known if there exist solutions to sigma(k) = 2k+1.
Sequence also gives the nonprime solutions to sigma(k) == 0 (mod k+1), k > 1.  Benoit Cloitre, Feb 05 2002
Sequence seems to give nonprime k such that the numerator of the sum of the reciprocals of the divisors of k equals k+1 (nonprime k such that A017665(k)=k+1).  Benoit Cloitre, Apr 04 2002
a(12) > 10^12.  Donovan Johnson, Mar 01 2012
For k > 1, composite numbers k such that A108775(k) = floor(sigma(k)/k) = sigma(k) mod k = A054024(k). Complement of primes (A000040) with respect to A230606. There are no numbers k > 2 such that sigma(x) = k*(x+1) has a solution.  Jaroslav Krizek, Dec 05 2013
a(13) > 10^13.  Giovanni Resta, Apr 02 2014
a(14) <= 144115187270549504 = 2^28*(2^29  3).  Jon E. Schoenfield, Jun 02 2019


REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, B2.


LINKS

Table of n, a(n) for n=1..13.
Amitabha Tripathi, A note on products of primes that differ by a fixed integer, Fibonacci Quart. 48 (2010), no. 2, 144149.


EXAMPLE

sigma(650) = 1302 == 2 (mod 650).


MATHEMATICA

Do[If[Mod[DivisorSigma[1, n]2, n]==0, Print[n]], {n, 1, 10^8}]
Join[{1}, Select[Range[8000000], Mod[DivisorSigma[1, #], #]==2 &]] (* Vincenzo Librandi, Mar 11 2014 *)


PROG

(PARI) is(n)=sigma(n)%n==2  n==1 \\ Charles R Greathouse IV, Mar 09 2014


CROSSREFS

Numbers k such that A054013(k)=1.
Cf. A181597, A050414, A050415, A054024, A045769, A088834, A045770, A076496.
Sequence in context: A241355 A220207 A189950 * A088831 A063785 A181703
Adjacent sequences: A045765 A045766 A045767 * A045769 A045770 A045771


KEYWORD

nonn


AUTHOR

Dan Hoey


EXTENSIONS

More terms from Jud McCranie, Dec 22 1999. He says there are no other terms < 4290000000.
a(11) from Donovan Johnson, Mar 01 2012
a(12) from Giovanni Resta, Apr 02 2014
a(13) from Jud McCranie, Jun 02 2019


STATUS

approved



