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 A067351 #16 Aug 18 2025 18:34:13 %S A067351 3,5,6,7,10,11,13,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,33, %T A067351 35,37,39,40,41,42,43,44,46,47,49,51,52,53,55,56,57,58,59,60,61,64,66, %U A067351 67,68,71,72,73,75,76,78,79,80,81,82,83,84,85,87,89,91,92,93,95,96,97 %N A067351 Numbers k such that sigma(k) + phi(k) has exactly 2 distinct prime divisors. %H A067351 G. C. Greubel, <a href="/A067351/b067351.txt">Table of n, a(n) for n = 1..1000</a> %F A067351 a(n) = A001221(A000010(n) + A000203(n)) = A001221(A065387(n)) = 2. %e A067351 Includes all odd primes and some composites; e.g., 21 and 25, since sigma(21) + phi(21) = 32 + 12 = 44 = 2*2*11; sigma(25) + phi(25) = 31 + 20 = 51 = 3*17. %t A067351 Select[ Range[ 1, 100 ], Length[ FactorInteger[ DivisorSigma[ 1, # ]+EulerPhi[ # ] ] ]==2& ] %t A067351 Select[Range[500], PrimeNu[EulerPhi[#] + DivisorSigma[1, #]] == 2 &] (* _G. C. Greubel_, May 08 2017 *) %Y A067351 Cf. A000005, A000010, A000203, A001221, A065387, A067349, A067350. %K A067351 nonn %O A067351 1,1 %A A067351 _Labos Elemer_, Jan 17 2002 %E A067351 Edited by _Dean Hickerson_, Jan 20 2002