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Positive integers not of the form sigma(k) + phi(k) - 2k for some k.
5

%I #21 Dec 02 2016 01:11:03

%S 5,11,19,21,23,27,33,35,39,43,45,47,51,53,55,59,67,69,71,79,81,83,87,

%T 95,99,101,103,105,107,113,115,117,119,123,129,131,135,139,141,143,

%U 147,151,153,155,159,165,167,171,173,175,177,179,185,187,189,191,193,195

%N Positive integers not of the form sigma(k) + phi(k) - 2k for some k.

%C Under the assumption that every even integer >= 18 is the sum of three distinct primes, all elements of this sequence are odd. See A278373 for more details.

%H David W. Wilson, <a href="/A056996/b056996.txt">Table of n, a(n) for n = 1..10000</a>

%t Take[#, 59] &@ Complement[Range@ #, Union@ Table[DivisorSigma[1, n] + EulerPhi@ n - 2 n, {n, #}]] &[10^5] (* _Michael De Vlieger_, Nov 30 2016 *)

%Y Complement is A278373.

%Y Cf. A000010 (phi), A000203 (sigma).

%K nonn

%O 1,1

%A _David W. Wilson_, Sep 06 2000