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 A248020 #27 Oct 05 2020 13:16:48 %S A248020 21,35,39,50,55,57,63,65,75,77,85,93,98,111,115,119,129,133,143,155, %T A248020 161,171,175,183,185,187,189,201,203,205,209,215,217,219,221,235,237, %U A248020 242,245,247,253,259,265,275,279,291,299,301,305,309,319,323,325,327,329,333,335,338,341 %N A248020 Numbers which are coprime to the sum of their divisors, but are neither primes nor perfect powers. %C A248020 Intersection of A003624 and A106543. - _Michel Marcus_, Sep 30 2014 %C A248020 Duffinian numbers (A003624) which are not perfect powers (A001597). - _Robert G. Wilson v_, Oct 02 2014 %H A248020 Amiram Eldar, <a href="/A248020/b248020.txt">Table of n, a(n) for n = 1..10000</a> %e A248020 21 is in the sequence since it is neither a prime nor a powerful number and its divisors 1, 3, 7, and 21 sum to 32, which is coprime to 21. %e A248020 50 is in the sequence since it is neither a prime nor a powerful number and its divisors 1, 2, 5, 10, 25, and 50 sum to 93, which is coprime to 50. %t A248020 perfectPowerQ[n_] := n == 1 || GCD @@ FactorInteger[n][[All, 2]] > 1; Select[ Range@ 350, !PrimeQ[ #] && GCD[#, DivisorSigma[1, #]] == 1 && !perfectPowerQ[ #] &] %t A248020 cpQ[n_]:=CoprimeQ[n,DivisorSigma[1,n]]&&!PrimeQ[n]&&GCD@@ FactorInteger[ n][[All,2]]<2; Select[Range[2,400],cpQ] (* _Harvey P. Dale_, Oct 05 2020 *) %o A248020 (PARI) forcomposite(n=1, 1e3, if(gcd(n, sigma(n))==1, if(!ispower(n), print1(n, ", ")))) \\ _Felix Fröhlich_, Oct 25 2014 %K A248020 nonn,easy %O A248020 1,1 %A A248020 _Robert G. Wilson v_, Sep 29 2014