A230203 - OEIS

%I #21 Apr 04 2024 02:16:33

%S 5,7,9,11,13,17,19,21,23,25,26,27,29,31,33,35,37,38,39,41,43,45,47,49,

%T 51,52,53,55,56,57,59,61,62,63,65,67,69,71,73,74,75,76,77,79,81,82,83,

%U 85,86,87,88,89,91,93,95,97,99,101,103,104,105,107,109,111,112,113,115,116,117,119

%N Numbers k such that sigma(phi(k)) > k.

%C It seems that the odd number k is not in the sequence if and only if k+2 is a Fermat prime (A019434).

%H Amiram Eldar, <a href="/A230203/b230203.txt">Table of n, a(n) for n = 1..10000</a>

%p for n do if sigma(phi(n))>n then print{n} fi od:

%t Select[Range[150],DivisorSigma[1,EulerPhi[#]]>#&] (* _Harvey P. Dale_, Apr 14 2019 *)

%o (PARI) is(k) = sigma(eulerphi(k)) > k; \\ _Amiram Eldar_, Apr 04 2024

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

%Y Cf. A230201, A001229, A018784, A019434, A066694, A227011, A227927, A062402, A230203, A018784.

%K nonn

%O 1,1

%A _Vladimir Letsko_, Oct 11 2013