A066811 Numbers k such that the concatenation of odd numbers from 1 to k is a prime.

3, 19, 31, 67, 97, 5139

Offset: 1

Author

Patrick De Geest, Jan 20 2002

Keywords

• nonn
• base
• more
• changed

Comments

a(7) > 50000. - Michael S. Branicky, Aug 28 2025

Examples

			19 is a term because 135791113151719 is a prime.
		

Crossrefs

Cf. A000040, A005408, A046036, A048847.

Programs

Mathematica

p = ""; Do[p = p <> ToString[2*n+1]; If[PrimeQ[ToExpression[p]], Print[2*n+1]], {n, 0, 2569}] (* Ryan Propper, Aug 26 2005 *)

Python

from sympy import isprime
def agen():
  k, str1tok = 1, '1'
  while True:
    if isprime(int(str1tok)): yield k
    k, str1tok = k + 2, str1tok + str(k + 2)
g = agen()
print([next(g) for i in range(5)]) # Michael S. Branicky, Mar 19 2021

Formula

a(n) = 2*A046036(n) - 1. - Michel Marcus, Jan 31 2014

Extensions

a(6) from Ryan Propper, Aug 26 2005