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 A300869 Odd numbers m such that sigma(x) = m has more than 1 solution. 5

%I

%S 31,399,403,1767,3751,4123,5187,5673,9517,11811,12369,17143,22971,

%T 27001,30783,33883,34671,43617,48279,53413,53599,54873,58683,68859,

%U 69967,73017,73749,80199,86831,88753,109771,117273,122493,123721,141267,152019,153543,158503,160797

%N Odd numbers m such that sigma(x) = m has more than 1 solution.

%C Goormaghtigh conjecture implies that 31 is the only prime in this sequence. - _Jianing Song_, Apr 27 2019

%H Robert Israel, <a href="/A300869/b300869.txt">Table of n, a(n) for n = 1..9260</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Goormaghtigh_conjecture">Goormaghtigh conjecture</a>

%e a(1) = 31 = A123523(2), the smallest odd number m for which sigma(x) = m has (at least, and also exactly) two solutions, x = 16 and x = 25.

%e a(56) = 347529 = A123523(3) is the smallest odd m for which sigma(x) = m has (at least, and also exactly) three solutions, x = 406^2, x = 2*319^2 and x = 489^2.

%p N:= 200000: # for terms <= N

%p Res:= NULL: count:= 0:

%p for m from 1 to floor(sqrt(N)) by 2 do

%p sm:= numtheory:-sigma(m^2);

%p for k from 1 to floor(log(N/sm+1)) do

%p v:= sm*(2^k-1);

%p if v <= N then Res:= Res, v; count:= count+1 fi;

%p od

%p od:

%p B:= sort([Res]):

%p Dups:= select(t -> B[t+1]=B[t], [\$1..nops(B)-1]):

%p sort(convert(convert(B[Dups],set),list)); # _Robert Israel_, Jan 15 2020

%t With[{s = PositionIndex@ Array[DivisorSigma[1, #] &, 10^6]}, Keys@ KeySort@ KeySelect[s, And[OddQ@ #, Length@ Lookup[s, #] > 1] &]] (* _Michael De Vlieger_, Mar 16 2018 *)

%o (PARI) MAX=1e6; LIM=1e4; b=0; A300869=[]; for(x=1,LIM, for(i=1,2, (s=sigma(i*x^2))>MAX && next(2); bittest(b,s\2) && (setsearch(A300869,s) || S=setunion(A300869,[s])) || b+=1<<(s\2)))

%Y Odd terms in A159886.

%Y Cf. A000203 (sigma), A002191, A007368.

%Y A123523 is a subsequence, except for the initial 1.

%Y Cf. A331036.

%K nonn

%O 1,1

%A _M. F. Hasler_, following a suggestion from _Altug Alkan_, Mar 16 2018

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Last modified August 5 14:42 EDT 2021. Contains 346469 sequences. (Running on oeis4.)