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%I #21 Jan 17 2019 13:44:07
%S 1,5,11,14,62,74,107,110,121,128,139,320,2956,11116
%N Numbers n such that 5*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (47*10^n+61)/9 is prime.
%C a(15) > 10^5. - _Robert Price_, Jul 16 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/52229.htm#prime">Prime numbers of the form 522...229</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101576(n) + 1.
%t Do[ If[ PrimeQ[(46*10^n + 61)/9], Print[n]], {n, 0, 10000}]
%o (PARI) repunit(n)=my(r=0,i);for(i=1,n,r=10*r+1); r
%o f(n)=5*10^n + 2*repunit(n) + 7
%o main(size)=my(i,r=1,v=vector(size));for(i=1,size,while(1, if(isprime(f(r)),v[i]=r;r++;break,r++))); v; \\ _Anders Hellström_, Jul 16 2015
%Y Cf. A002275, A101576.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E Addition of a(14) from Kamada data by _Robert Price_, Dec 12 2010
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