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A045769 Numbers k such that sigma(k) == 4 (mod k). 10
1, 9, 12, 70, 88, 1888, 4030, 5830, 32128, 521728, 1848964, 8378368, 34359083008, 66072609790, 549753192448 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(16) > 10^12. - Donovan Johnson, Mar 01 2012

a(16) > 10^13. - Giovanni Resta, Apr 02 2014

a(16) > 1.5*10^14. - Jud McCranie, Jun 02 2019

a(16) <= 2251799645913088. Every number of the form 2^(j-1)*(2^j - 5), where 2^j - 5 is prime, is a term. - Jon E. Schoenfield, Jun 02 2019

LINKS

Table of n, a(n) for n=1..15.

MATHEMATICA

Join[{1}, Select[Range[9000000], Mod[DivisorSigma[1, #], #]==4 &]] (* Harvey P. Dale, Mar 20 2011 *)

PROG

(PARI) is(n)=sigma(n)%n==4 || n==1 \\ Charles R Greathouse IV, Mar 09 2014

CROSSREFS

Cf. A054024, A045768, A045769, A088834, A045770, A076496.

Sequence in context: A341208 A328120 A072308 * A140145 A166643 A038302

Adjacent sequences:  A045766 A045767 A045768 * A045770 A045771 A045772

KEYWORD

nonn

AUTHOR

Dan Hoey

EXTENSIONS

Additional term (a(11)) from Harvey P. Dale, Mar 20 2011

Initial term 1 inserted and a(13)-a(15) from Donovan Johnson, Mar 01 2012

Typo in Mathematica program fixed by Vincenzo Librandi, Mar 11 2014

STATUS

approved

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Last modified April 16 12:45 EDT 2021. Contains 343037 sequences. (Running on oeis4.)