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A117307
Numbers for which (phi(n))^2+phi(n)+1 is a prime number.
1
1, 2, 3, 4, 6, 7, 9, 13, 14, 15, 16, 18, 20, 21, 24, 25, 26, 28, 30, 33, 35, 36, 39, 42, 44, 45, 50, 52, 56, 66, 67, 70, 72, 78, 79, 81, 84, 90, 121, 123, 134, 139, 151, 158, 162, 163, 164, 165, 176, 193, 200, 203, 215, 220, 221, 242, 243, 245, 246, 249
OFFSET
1,2
LINKS
EXAMPLE
14 is in the sequence because (phi(14))^2+phi(14)+1 = 6^2+6+1 = 43, which is a prime number.
MATHEMATICA
f[x_] := x^2 + x + 1; Select[Range[250], PrimeQ[f[EulerPhi[#]]] &] (* Amiram Eldar, Feb 08 2021 *)
PROG
(PARI) lista(nn) = {for (n = 1, nn, if (isprime((eulerphi(n))^2 + eulerphi(n) + 1), print1(n, ", ")); ); } \\ Michel Marcus, Jun 01 2013
CROSSREFS
Sequence in context: A117595 A050050 A222801 * A340877 A161890 A089388
KEYWORD
nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), Apr 24 2006
EXTENSIONS
Corrected by Michel Marcus, Jun 01 2013
STATUS
approved