A090562 Primes of the form 5k^2 + 5k + 1.

11, 31, 61, 101, 151, 211, 281, 661, 911, 1051, 1201, 1361, 1531, 1901, 2311, 2531, 3001, 3251, 3511, 4651, 5281, 6301, 6661, 7411, 9461, 9901, 12251, 13781, 14851, 15401, 18301, 18911, 19531, 20161, 22111, 24151, 24851, 25561, 27011, 27751

Offset: 1

Author

Amarnath Murthy, Dec 11 2003

Keywords

• easy
• nonn

Comments

Or, primes obtained as a concatenation of a triangular number and 1.

Centered decagonal primes. - Paul Muljadi, Oct 04 2005

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..5000
• Wikipedia, Centered decagonal number.

Crossrefs

Cf. A090563, A062786.

Programs

Magma

[a: n in [0..100] | IsPrime(a) where a is 5*n^2 + 5*n + 1]; // Vincenzo Librandi, Dec 13 2011

Mathematica

Select[5(#^2 - #) + 1 & /@ Range[75], PrimeQ[ # ] &] (* Robert G. Wilson v, Oct 10 2005 *)

Extensions

Edited and extended by Robert G. Wilson v, Oct 10 2005