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A056724 Numbers n such that 8*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1

%I

%S 2,9,15,32,38,65,123,173,257,320,326,639,719,774,902,15210,69999

%N Numbers n such that 8*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 (79*10^n-7)/9 is prime.

%C a(18) > 10^5. - _Robert Price_, Jan 11 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/87777.htm#prime">Prime numbers of the form 877...77</a>.

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

%t Do[ If[ PrimeQ[ 8*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A093943.

%K hard,nonn

%O 1,1

%A _Robert G. Wilson v_, Aug 11 2000

%E a(16)-a(17) from _Robert Price_, Jan 11 2015

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Last modified June 24 17:31 EDT 2021. Contains 345417 sequences. (Running on oeis4.)