A067351 - OEIS

%I #13 Sep 04 2017 23:23:43

%S 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 35,37,39,40,41,42,43,44,46,47,49,51,52,53,55,56,57,58,59,60,61,64,66,

%U 67,68,71,72,73,75,76,78,79,80,81,82,83,84,85,87,89,91,92,93,95,96,97

%N Numbers n such that sigma(n) + phi(n) has exactly 2 distinct prime divisors.

%H G. C. Greubel, <a href="/A067351/b067351.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A001221(A000010(n) + A000203(n)) = A001221(A065387(n)) = 2.

%e 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 Select[ Range[ 1, 100 ], Length[ FactorInteger[ DivisorSigma[ 1, # ]+EulerPhi[ # ] ] ]==2& ]

%t Select[Range[500], PrimeNu[EulerPhi[#] + DivisorSigma[1, #]] == 2 &] (* _G. C. Greubel_, May 08 2017 *)

%Y Cf. A000005, A000010, A000203, A001221, A065387, A067349, A067350.

%K nonn

%O 1,1

%A _Labos Elemer_, Jan 17 2002

%E Edited by _Dean Hickerson_, Jan 20 2002