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 A331036 Odd values of the sum-of-divisors function sigma (A000203), listed by increasing size and with multiplicity. 2
 1, 3, 7, 13, 15, 31, 31, 39, 57, 63, 91, 93, 121, 127, 133, 171, 183, 195, 217, 255, 307, 363, 381, 399, 399, 403, 403, 465, 511, 549, 553, 741, 781, 819, 847, 855, 871, 921, 931, 961, 993, 1023, 1093, 1143, 1209, 1281, 1407, 1651, 1659, 1723, 1729, 1767, 1767, 1815, 1893, 1953 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS See A060657 for the range (without repeated terms) and A152677 for the subsequence of odd values in A000203. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 MAPLE N:= 2000: # for terms <= N Res:= NULL: 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: sort([Res]); # Robert Israel, Jan 14 2020 MATHEMATICA Sort@ Select[DivisorSigma[1, Range@ 2000], OddQ[#] && # < 2000 &] (* Giovanni Resta, Jan 08 2020 *) PROG (PARI) list(lim)=select(k->k<=lim, vecsort(apply(sigma, concat(vector(sqrtint(lim\1), i, i^2), vector(sqrtint(lim\2), i, 2*i^2))))) \\ Charles R Greathouse IV, Feb 15 2013 [originally added in A152677] CROSSREFS Cf. A060657, A023195 (subset of primes), A152677 (subsequence of odd values in A000203), A300869 (repeated terms). Sequence in context: A080565 A164344 A002254 * A060657 A192148 A151875 Adjacent sequences:  A331033 A331034 A331035 * A331037 A331038 A331039 KEYWORD nonn AUTHOR M. F. Hasler, Jan 08 2020 STATUS approved

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Last modified July 28 10:29 EDT 2021. Contains 346326 sequences. (Running on oeis4.)