A109137 - OEIS

%I #20 Oct 17 2024 05:50:52

%S 2,14,42,144,200,302,7242,8718,10568,24438,41734

%N Numbers k such that k * (10^k - 1) + 1 is prime.

%C Larger values certified with ECM. No more up to 6000.

%o (PARI) is(n)=ispseudoprime(n*(10^n-1)+1) \\ _Charles R Greathouse IV_, Jun 13 2017

%o (Python)

%o from sympy import isprime

%o def afind(limit, startk=1):

%o     k, pow10 = startk, 10**startk

%o     for k in range(startk, limit+1):

%o         if isprime(k*(pow10 - 1) + 1): print(k, end=", ")

%o         k += 1

%o         pow10 *= 10

%o afind(500) # _Michael S. Branicky_, Aug 26 2021

%Y Cf. A110807.

%K more,nonn

%O 1,1

%A _Jason Earls_, Aug 18 2005

%E a(7)-a(8) from _Ryan Propper_, Sep 20 2006

%E a(9) from _Michael S. Branicky_, Aug 26 2021

%E a(10) from _Michael S. Branicky_, Apr 05 2023

%E a(11) from _Michael S. Branicky_, Oct 17 2024