OFFSET
1,1
COMMENTS
Primes in this sequence are of the form 2*m^2+1 (see A090698). - Bernard Schott, Mar 07 2020
If k is an odd term, so is 2*k. If k is an even term, so is 4*k. - Waldemar Puszkarz, Oct 15 2024
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
a(4) is 15 because phi(15) = 8, which is twice the square of 2.
MATHEMATICA
Select[Range[500], IntegerQ @ Sqrt[EulerPhi[#]/2] &] (* Amiram Eldar, Mar 07 2020 *)
PROG
(PARI) isok(n) = issquare(eulerphi(n)/2) \\ Michel Marcus, Jul 23 2013
(Python)
from sympy import totient
from sympy.ntheory.primetest import is_square
for i in range(3, 501):
if is_square(int(totient(i)/2)):
print(i, end=", ") # Waldemar Puszkarz, Oct 15 2024
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
Walter Nissen, May 20 2007
STATUS
approved