OFFSET
1,2
COMMENTS
Also numbers n such that A270390(n) = 1.
Conjectured to be infinite: see the Ailon and Rudnick paper.
LINKS
N. Ailon and Z. Rudnick, Torsion points on curves and common divisors of a^k - 1 and b^k - 1 , Acta Arith. 113 (2004), pp. 31-38.
EXAMPLE
gcd(2^1-1, 5^1-1) = gcd(1,4) = 1, so a(1) = 1.
gcd(2^3-1, 5^3-1) = gcd(7,124) = 1, so a(2) = 3.
gcd(2^4-1, 5^4-1) = gcd(15,624) = 3, so 4 is not in the sequence.
MATHEMATICA
Select[Range[200], GCD[2^# - 1, 5^# - 1] == 1 &]
Select[Range[150], CoprimeQ[2^#-1, 5^#-1]&] (* Harvey P. Dale, Apr 12 2018 *)
PROG
(Magma) [n: n in [1..200] | Gcd(2^n-1, 5^n-1) eq 1];
CROSSREFS
KEYWORD
nonn
AUTHOR
Vincenzo Librandi, May 01 2016
STATUS
approved