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 A056100 #24 Aug 13 2025 10:23:29
%S A056100 0,0,0,3,0,1,0,5,7,3,0,5,0,5,13,9,0,1,0,17,7,9,0,1,21,11,19,1,0,7,0,
%T A056100 17,4,15,33,13,0,17,19,1,0,19,0,9,28,21,0,17,43,11,10,13,0,1,21,25,31,
%U A056100 27,0,49,0,29,28,33,3,43,0,21,16,27,0,1,0,35,11,25,63,55,0,33,55,39,0,1
%N A056100 a(n) = sigma(n)*phi(n) + 1 (mod n).
%C A056100 Note that iff p is a prime then sigma(p)*phi(p) + 1 = 0 (mod p).
%D A056100 George E. Andrews, "Number Theory," Dover Publ., NY, 1971, page 85.
%H A056100 Karl-Heinz Hofmann, <a href="/A056100/b056100.txt">Table of n, a(n) for n = 1..10000</a>
%t A056100 Table[Mod[DivisorSigma[1, n]*EulerPhi[n] + 1, n], {n, 1, 100}]
%o A056100 (PARI) a(n) = (sigma(n)*eulerphi(n)+1) % n; \\ _Michel Marcus_, Aug 05 2025
%o A056100 (Python)
%o A056100 from sympy import totient, divisor_sigma
%o A056100 def A056100(n): return (totient(n)*divisor_sigma(n)+1)%n # _Karl-Heinz Hofmann_, Aug 12 2025
%Y A056100 Cf. A000010, A000203, A062354.
%K A056100 easy,nonn
%O A056100 1,4
%A A056100 _Robert G. Wilson v_, Jul 28 2000