A097009 - OEIS

%I #16 Sep 08 2022 08:45:14

%S 1,2,4,6,2,12,2,2,2,14,6,4,20,2,2,26,2,30,6,2,36,6,2,2,12,2,2,2,54,56,

%T 2,10,4,6,4,2,12,2,2,4,2,36,2,24,14,2,70,2,2,12,4,2,48,2,8,2,2,10,2,

%U 14,6,2,2,2,12,2,6,48,2,4,4,2,2,4,2,2,2,2,16,4,2,10,2,12,2,34,8,8,20,6,2,2

%N a(n) = gcd(prime(2*n)-1, prime(n)-1).

%H Michael De Vlieger, <a href="/A097009/b097009.txt">Table of n, a(n) for n = 1..10000</a>

%e a(3) = gcd(Prime(6)-1, prime(3)-1) = gcd(13-1, 5-1) = gcd(12, 4) = 4.

%t Table[GCD[Prime[2 n] - 1, Prime@ n - 1], {n, 92}] (* _Michael De Vlieger_, Dec 03 2015 *)

%o (Magma) [Gcd(NthPrime(2*n)-1, NthPrime(n)-1): n in [1..80]]; // _Vincenzo Librandi_, Dec 04 2015

%o (PARI) a(n) = gcd(prime(2*n)-1, prime(n)-1); \\ _Michel Marcus_, Dec 04 2015

%Y Cf. A066978 (gcd(prime(2*n)+1, prime(n)+1)).

%K nonn

%O 1,2

%A _Labos Elemer_, Jul 26 2004