A180645 - OEIS

%I #8 Feb 15 2014 15:46:56

%S 7,9,13,14,18,21,26,28,36,37,42,57,63,65,73,74,76,79,91,95,97,104,105,

%T 108,111,112,114,117,119,126,127,130,135,140,144,146,148,152,153,156,

%U 158,163,168,180,182,190,194,195,199,203,208,210,215,216,222,224,228

%N Numbers n such that 5+phi(n)^2 is prime. Phi(n) is Euler's totient.

%C The sequence appears to be infinite, but I have no proof.

%C There are many consecutive elements in the sequence: (13,14)-(36,37)-...-(12777,12778)-...(30236,30237)-...

%e a(10)=37 since 5+phi(37)^2=1301 is a prime.

%t Select[Range[250],PrimeQ[EulerPhi[#]^2+5]&] (* _Harvey P. Dale_, Jun 07 2011 *)

%Y Cf. A000010.

%K nonn

%O 1,1

%A _Carmine Suriano_, Sep 14 2010