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 A300869 Odd numbers m such that sigma(x) = m has more than 1 solution. 5
 31, 399, 403, 1767, 3751, 4123, 5187, 5673, 9517, 11811, 12369, 17143, 22971, 27001, 30783, 33883, 34671, 43617, 48279, 53413, 53599, 54873, 58683, 68859, 69967, 73017, 73749, 80199, 86831, 88753, 109771, 117273, 122493, 123721, 141267, 152019, 153543, 158503, 160797 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Goormaghtigh conjecture implies that 31 is the only prime in this sequence. - Jianing Song, Apr 27 2019 LINKS Robert Israel, Table of n, a(n) for n = 1..9260 Wikipedia, Goormaghtigh conjecture EXAMPLE 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. 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. MAPLE N:= 200000: # for terms <= N Res:= NULL: count:= 0: for m from 1 to floor(sqrt(N)) by 2 do sm:= numtheory:-sigma(m^2); for k from 1 to floor(log[2](N/sm+1)) do    v:= sm*(2^k-1);    if v <= N then Res:= Res, v; count:= count+1 fi; od od: B:= sort([Res]): Dups:= select(t -> B[t+1]=B[t], [\$1..nops(B)-1]): sort(convert(convert(B[Dups], set), list)); # Robert Israel, Jan 15 2020 MATHEMATICA With[{s = PositionIndex@ Array[DivisorSigma[1, #] &, 10^6]}, Keys@ KeySort@ KeySelect[s, And[OddQ@ #, Length@ Lookup[s, #] > 1] &]] (* Michael De Vlieger, Mar 16 2018 *) PROG (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))) CROSSREFS Odd terms in A159886. Cf. A000203 (sigma), A002191, A007368. A123523 is a subsequence, except for the initial 1. Cf. A331036. Sequence in context: A328750 A179465 A142829 * A297019 A022691 A125443 Adjacent sequences:  A300866 A300867 A300868 * A300870 A300871 A300872 KEYWORD nonn AUTHOR M. F. Hasler, following a suggestion from Altug Alkan, Mar 16 2018 STATUS approved

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Last modified June 18 11:58 EDT 2021. Contains 345098 sequences. (Running on oeis4.)