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A097018 a(n) is the least number such that sigma(a(n)) is divisible by the n-th prime. 4
3, 2, 8, 4, 43, 9, 67, 37, 137, 173, 16, 73, 163, 257, 281, 211, 353, 169, 401, 283, 256, 157, 331, 1024, 193, 1009, 617, 641, 653, 677, 64, 523, 547, 277, 1489, 1811, 313, 977, 1669, 691, 1789, 1447, 4201, 1543, 787, 397, 421, 1783, 907, 457, 3727, 1433, 3373 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Note that a(n) always exists, because sigma(2^(p-2)) = 2^(p-1)-1 is divisible by p for p>2 (by Fermat's little theorem), so there is always a candidate for a(n). Compare A227470, A272349. - N. J. A. Sloane, May 01 2016

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

EXAMPLE

n=11: a(11)=16 is the least number x such that sigma(x) is divisible by the 11th prime, 31.

MATHEMATICA

ln[n_]:=Module[{x=1, p=Prime[n]}, While[!Divisible[DivisorSigma[ 1, x], p], x++]; x]; Array[ln, 60] (* Harvey P. Dale, Sep 07 2014 *)

Module[{nn=5000, ds}, ds=DivisorSigma[1, Range[nn]]; Table[Position[ds, _?(Divisible[#, n]&), 1, 1], {n, Prime[Range[60]]}]]//Flatten (* Much faster than the first program *) (* Harvey P. Dale, May 18 2018 *)

PROG

(PARI) sigma_hunt(x)=local(n=0, g); while(n++, g=sigma(n); if(g%x, , return(n)));

for(x=1, 50, print1(sigma_hunt(prime(x))", ")) /* Phil Carmody, Mar 01 2013 */

(MAGMA) sol:=[]; p:=PrimesUpTo(10000); for n in [1..53] do k:=2; while Max(PrimeDivisors(SumOfDivisors(k))) ne p[n] do k:=k+1; end while; sol[n]:=k; end for; sol; // Marius A. Burtea, Jun 05 2019

CROSSREFS

Cf. A000203, A000040, A272349, A227470.

Sequence in context: A127300 A129199 A211164 * A127541 A053219 A173030

Adjacent sequences:  A097015 A097016 A097017 * A097019 A097020 A097021

KEYWORD

nonn

AUTHOR

Labos Elemer, Aug 23 2004

STATUS

approved

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Last modified October 19 15:50 EDT 2019. Contains 328223 sequences. (Running on oeis4.)