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Numbers n such that 8*10^n - 9 is prime.
1

%I #21 Jan 17 2019 13:44:07

%S 1,5,11,12,17,28,46,48,61,73,138,178,213,405,762,1053,1157,1427,2865,

%T 3148,3615,13447,48081

%N Numbers n such that 8*10^n - 9 is prime.

%C Also numbers n such that 7*10^n + 9*R_n - 8 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C a(24) > 2*10^5. - _Robert Price_, Dec 19 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79991.htm#prime">Prime numbers of the form 799...991</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = A101154(n) + 1.

%t Do[ If[ PrimeQ[8*10^n - 9], Print[n]], {n, 0, 10000}]

%o (PARI) is(n)=ispseudoprime(8*10^n-9) \\ _Charles R Greathouse IV_, Jun 12 2017

%Y Cf. A002275, A101154.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 19 2005

%E a(22)-a(23) from Kamada data by _Robert Price_, Dec 14 2010