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A056727 Numbers n such that 9*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1

%I

%S 1,2,4,19,28,73,203,220,274,292,470,763,1891,3307,7007,7306,9755,

%T 11395,39452,78242

%N Numbers n such that 9*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (88*10^n-7)/9 is prime.

%C a(21) > 10^5. - _Robert Price_, Nov 30 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/97777.htm#prime">Prime numbers of the form 977...77</a>.

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

%t Do[ If[ PrimeQ[ 9*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A093944 (corresponding primes).

%K hard,nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 11 2000

%E a(18)-a(20) from _Robert Price_, Nov 30 2014

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Last modified May 17 22:15 EDT 2021. Contains 343992 sequences. (Running on oeis4.)