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A286917 Numbers k such that there is an anti-divisor d of k satisfying sigma(d) = k. 0
3, 4, 13, 32, 40, 60, 121, 364, 1093, 3200, 3280, 9841, 15120, 16380, 29282, 29524, 88573, 91728, 264992, 265720, 797161 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..21.

EXAMPLE

Anti-divisors of 60 are 7, 8, 11, 17, 24, 40 and sigma(24) = 60.

MAPLE

with(numtheory): P:= proc(q) local a, k, n; for n from 3 to q do a:=[];

for k from 2 to n-1 do if abs((n mod k)-k/2)<1 then a:=[op(a), k]; fi; od;

for k from 1 to nops(a) do if n=sigma(a[k]) then print(n); break; fi; od;

od; end: P(10^4); # Paolo P. Lava, May 16 2017

PROG

(PARI) isok(n) = {ad = select(t->n%t && t<n, concat(concat(divisors(2*n-1), divisors(2*n+1)), 2*divisors(n))); for (k=1, #ad, if ((n % ad[k]) && (sigma(ad[k])== n), return (1)); ); } \\ Michel Marcus, May 20 2017

CROSSREFS

Cf. A000203, A066272, A081756, A130799.

Sequence in context: A192872 A036672 A174684 * A084315 A194649 A062165

Adjacent sequences:  A286914 A286915 A286916 * A286918 A286919 A286920

KEYWORD

nonn,more,changed

AUTHOR

Paolo P. Lava, May 16 2017

EXTENSIONS

More terms from Michel Marcus, May 20 2017

STATUS

approved

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Last modified September 15 18:22 EDT 2019. Contains 327082 sequences. (Running on oeis4.)