A087349 a(n) = n + (smallest prime-factor of n+1).

3, 5, 5, 9, 7, 13, 9, 11, 11, 21, 13, 25, 15, 17, 17, 33, 19, 37, 21, 23, 23, 45, 25, 29, 27, 29, 29, 57, 31, 61, 33, 35, 35, 39, 37, 73, 39, 41, 41, 81, 43, 85, 45, 47, 47, 93, 49, 55, 51, 53, 53, 105, 55, 59, 57, 59, 59, 117, 61, 121, 63, 65, 65, 69, 67, 133, 69, 71, 71

Offset: 1

Author

Reinhard Zumkeller, Oct 20 2003

Keywords

• nonn
• easy

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A020639, A061228.

Programs

Haskell

a087349 n = a020639 (n + 1) + n  -- Reinhard Zumkeller, Mar 31 2014

Mathematica

a[n_] := n + FactorInteger[n+1][[1,1]]; Array[a, 100] (* Amiram Eldar, Apr 10 2025 *)

PARI

a(n) = n + factor(n+1)[1,1]; \\ Amiram Eldar, Apr 10 2025

Formula

a(n) = n + A020639(n+1).

a(n) = A061228(n+1) - 1.