A180645 Numbers n such that 5+phi(n)^2 is prime. Phi(n) is Euler's totient.

7, 9, 13, 14, 18, 21, 26, 28, 36, 37, 42, 57, 63, 65, 73, 74, 76, 79, 91, 95, 97, 104, 105, 108, 111, 112, 114, 117, 119, 126, 127, 130, 135, 140, 144, 146, 148, 152, 153, 156, 158, 163, 168, 180, 182, 190, 194, 195, 199, 203, 208, 210, 215, 216, 222, 224, 228

Offset

1,1

Comments

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

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

Links

Table of n, a(n) for n=1..57.

Example

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

Mathematica

Select[Range[250], PrimeQ[EulerPhi[#]^2+5]&] (* Harvey P. Dale, Jun 07 2011 *)

Crossrefs

Cf. A000010.

Sequence in context: A066962 A067020 A051913 * A196091 A329166 A222945

Adjacent sequences: A180642 A180643 A180644 * A180646 A180647 A180648

Keyword

nonn

Author

Carmine Suriano, Sep 14 2010

Status

approved