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

3, 19, 31, 67, 97, 5139

Offset

1,1

Links

Table of n, a(n) for n=1..6.

Formula

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

Example

19 is a term because 135791113151719 is a prime.

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 *)

Prog

(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

Crossrefs

Cf. A000040, A005408, A048847.

Sequence in context: A236969 A222590 A107165 * A269414 A162307 A128069

Adjacent sequences: A066808 A066809 A066810 * A066812 A066813 A066814

Keyword

base,nonn

Author

Patrick De Geest, Jan 20 2002

Extensions

a(6) from Ryan Propper, Aug 26 2005

Status

approved