cp's OEIS Frontend

A260626 a(n) = gcd(m, 2^m-1) where m is the n-th nonprime positive integer.

%I A260626 #37 Aug 25 2019 09:01:51
%S A260626 1,1,3,1,1,1,3,1,1,1,9,5,7,1,3,1,1,1,1,3,1,1,1,1,9,1,1,5,21,1,1,1,3,1,
%T A260626 1,1,1,27,1,1,1,1,15,1,7,1,1,3,1,1,1,9,1,1,1,1,3,5,1,1,21,1,1,1,1,9,1,
%U A260626 1,1,1,1,3,1,1,25,3,1,7,1,27,11,1,1,3,1
%N A260626 a(n) = gcd(m, 2^m-1) where m is the n-th nonprime positive integer.
%C A260626 2^m - 1 is a nonprime number if m is a nonprime number.
%H A260626 Michel Lagneau, <a href="/A260626/b260626.txt">Table of n, a(n) for n = 1..10000</a>
%F A260626 a(n) = gcd(A018252(n), 2^A018252(n)-1).
%F A260626 a(n) = A014491(A018252(n)). - _Michel Marcus_, Nov 01 2015
%p A260626 seq(`if`(isprime(m), NULL, igcd(m, 2^m-1)), m=1..150);
%t A260626 GCDnonPrime[n_Integer]:=GCD[2^FixedPoint[n+PrimePi@#&,n+PrimePi@n]-1,FixedPoint[n+PrimePi@#&,n+PrimePi@n]];Array[GCDnonPrime,120]
%t A260626 GCD[#,2^#-1]&/@Select[Range[200],!PrimeQ[#]&] (* _Harvey P. Dale_, Aug 25 2019 *)
%o A260626 (PARI) for(n=1, 1e3, if(!isprime(n), print1(gcd(n,2^n-1)", "))) \\ _Altug Alkan_, Nov 01 2015
%Y A260626 Cf. A014491, A018252.
%K A260626 nonn,easy
%O A260626 1,3
%A A260626 _Michel Lagneau_, Oct 31 2015