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Numbers k such that 10^k - k is prime.
9

%I #21 Mar 23 2021 16:08:07

%S 3,23,171,903,9911,48107,48449,60959

%N Numbers k such that 10^k - k is prime.

%C For each n, a(n) == 3 (mod 6) or a(n) == 5 (mod 6).

%C There is no further term up to 16000. - _Farideh Firoozbakht_, Dec 02 2006

%C a(9) > 10^5. - _Robert Price_, Feb 19 2015

%o (PARI) is(n)=isprime(10^n-n) \\ _Charles R Greathouse IV_, Feb 17 2017

%o (Python)

%o from sympy import isprime

%o def afind(limit):

%o m, pow10 = 0, 1

%o while m <= limit:

%o if isprime(pow10 - m): print(m, end=", ")

%o m, pow10 = m + 1, pow10 * 10

%o afind(1000) # _Michael S. Branicky_, Mar 23 2021

%Y Cf. A024115 (10^n-n).

%Y Cf. A110066, A089379, A110067, A110068, A174175, A174176, A174177, A174178, A193881, A193882.

%K more,nonn

%O 1,1

%A _Farideh Firoozbakht_, Jul 11 2005

%E a(5) from _Farideh Firoozbakht_, Dec 02 2006

%E Definition corrected by _Farideh Firoozbakht_, Dec 12 2006

%E a(6)-a(8) from _Robert Price_, Feb 19 2015