OFFSET
1,1
COMMENTS
First 5 Fermat primes from A019434 are terms of this sequence.
a(2) = 5 is only term of a(n) such that a(n) - 2 is a prime q, i.e., prime 3 is only prime q such that phi(q) = phi(q+1).
If there are any other Fermat primes, they will not be in the sequence. - Robert Israel, Mar 31 2015
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..819 (terms below 10^13, calculated from the b-file at A001274)
EXAMPLE
Prime 17 is in the sequence because phi(15) = phi(16) = 8.
MAPLE
with(numtheory): A256510:=n->`if`(isprime(n) and phi(n-2) = phi(n-1), n, NULL): seq(A256510(n), n=1..10^5); # Wesley Ivan Hurt, Mar 31 2015
MATHEMATICA
Select[Prime@ Range@ 100000, EulerPhi[# - 2] == EulerPhi[# - 1] &] (* Michael De Vlieger, Mar 31 2015 *)
PROG
(Magma) [n: n in [3..10^7] | IsPrime(n) and EulerPhi(n-2) eq EulerPhi(n-1)]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Mar 31 2015
STATUS
approved