The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A265166 Numbers n such that 2^n-1 and 5^n-1 are coprime. 0
 1, 3, 5, 7, 9, 11, 13, 17, 19, 21, 23, 25, 27, 29, 31, 33, 37, 41, 43, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 77, 79, 81, 83, 85, 87, 89, 91, 93, 97, 101, 103, 107, 109, 111, 113, 115, 121, 123, 125, 127, 129, 131, 133, 137, 139, 141, 143 (list; graph; refs; listen; history; text; internal format)
 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 Cf. A000225, A024049, A263647, A270390. Sequence in context: A324761 A244579 A151991 * A336620 A318978 A327755 Adjacent sequences:  A265163 A265164 A265165 * A265167 A265168 A265169 KEYWORD nonn AUTHOR Vincenzo Librandi, May 01 2016 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 20 08:03 EDT 2022. Contains 353852 sequences. (Running on oeis4.)