A058005 a(n) = gcd(2*n, binomial(2*n, n)).

2, 2, 2, 2, 2, 12, 2, 2, 2, 4, 2, 4, 2, 4, 30, 2, 2, 12, 2, 20, 6, 4, 2, 12, 2, 4, 2, 56, 2, 4, 2, 2, 6, 4, 14, 4, 2, 4, 2, 20, 2, 84, 2, 8, 90, 4, 2, 12, 2, 4, 6, 8, 2, 12, 2, 8, 6, 4, 2, 24, 2, 4, 6, 2, 10, 132, 2, 4, 6, 20, 2, 36, 2, 4, 30, 8, 154, 12, 2, 20, 2, 4, 2, 56, 10, 4, 6, 88, 2, 20

Offset: 1

Author

Labos Elemer, Nov 13 2000

Keywords

• nonn

Comments

a(n) = 2 for values like 1,2,3,4,5,7, ...; a(n) = 2n for values like 6,15,...,190.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001405, A071416.

Programs

Mathematica

a[n_] := GCD[2*n, Binomial[2*n, n]]; Array[a, 100] (* Amiram Eldar, Mar 07 2025 *)

PARI

a(n) = gcd(2*n, binomial(2*n, n)); \\ Amiram Eldar, Mar 07 2025

Extensions

Name corrected by Hugo Pfoertner, Mar 22 2020