This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A060657 #18 Dec 26 2024 03:53:15 %S A060657 1,3,7,13,15,31,39,57,63,91,93,121,127,133,171,183,195,217,255,307, %T A060657 363,381,399,403,465,511,549,553,741,781,819,847,855,871,921,931,961, %U A060657 993,1023,1093,1143,1209,1281,1407,1651,1659,1723,1729,1767,1815,1893,1953 %N A060657 Odd values of the sum-of-divisors function sigma (A000203) (without repetitions). %C A060657 That is, the odd values produced by the sigma function. %C A060657 Odd terms of A002191. - _Michel Marcus_, Jun 10 2014 %H A060657 Giovanni Resta, <a href="/A060657/b060657.txt">Table of n, a(n) for n = 1..10000</a> %H A060657 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp). %e A060657 a(7) = 39 because sigma(18) = 1+2+3+6+9+18 = 39, an odd number. %t A060657 nn = 2000; Union[Select[DivisorSigma[1, Range[nn]], OddQ[#] && # <= nn &]] (* _Harvey P. Dale_, Mar 12 2011 *) %o A060657 (PARI) is(k) = k % 2 && invsigmaNum(k) > 0; \\ _Amiram Eldar_, Dec 26 2024, using _Max Alekseyev_'s invphi.gp %Y A060657 Cf. A000203, A002191, A007369. %K A060657 nonn %O A060657 1,2 %A A060657 _Robert G. Wilson v_, Apr 18 2001 %E A060657 Name edited by _Giovanni Resta_, Jan 08 2020