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 A061367 #14 Jun 14 2025 11:05:54
%S A061367 15,35,95,119,143,209,287,319,323,357,377,527,559,779,899,923,989,
%T A061367 1007,1045,1189,1199,1343,1349,1763,1919,2159,2261,2507,2639,2759,
%U A061367 2911,3239,3339,3553,3599,3827,4031,4147,4607,5049,5183,5207,5249,5459,5543,6439
%N A061367 Composite n such that sigma(n)-phi(n) divides sigma(n)+phi(n).
%C A061367 Primes trivially satisfy the defining condition.
%H A061367 Amiram Eldar, <a href="/A061367/b061367.txt">Table of n, a(n) for n = 1..10000</a>
%F A061367 It seems that a(n) is asymptotic to c*n^2, 2<c<2.5 and that a(n)>2*n^2. - _Benoit Cloitre_, Sep 17 2002
%e A061367 sigma(15)-phi(15) = 24-8 = 16 divides sigma(15)-phi(15)=24+8 = 32, so 15 is a term of the sequence.
%t A061367 f[n_] := Module[{a = DivisorSigma[1, n], b = EulerPhi[n]}, Mod[(a + b), (a - b)] == 0]; Select[Range[2, 10^4], (f[ # ] && ! PrimeQ[ # ]) &]
%t A061367 cnQ[n_]:=With[{s=DivisorSigma[1,n],p=EulerPhi[n]},Mod[s+p,s-p]==0]; Select[Range[6500],CompositeQ[#]&&cnQ[#]&] (* _Harvey P. Dale_, Jun 14 2025 *)
%Y A061367 Cf. A051612, A065387.
%K A061367 nonn
%O A061367 1,1
%A A061367 _Joseph L. Pe_, Feb 13 2002