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
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, following a suggestion from Altug Alkan, Mar 16 2018
STATUS
approved