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%I #27 Jun 22 2018 08:09:14
%S 1,7,709,2203,4481,5107,11489
%N Numbers k such that k 1's followed by k is a prime.
%C The number corresponding to a(3) has been certified prime with Primo; those corresponding to a(4), a(5), a(6) are probable primes.
%C a(8) > 147000, if it exists. - _Giovanni Resta_, Jun 22 2018
%e 7 is in the sequence because 11111117 (seven 1's followed by 7) is prime.
%t Do[ If[ PrimeQ[ FromDigits[ Join[ IntegerDigits[(10^n - 1)/9], IntegerDigits[n]]]], Print[n]], {n, 1, 2000, 2}]
%o (WinPFGW) Primality testing r(5107)*10^len(5107)+5107 [N-1/N+1, Brillhart-Lehmer-Selfridge] r(5107)*10^len(5107)+5107 is Fermat and Lucas PRP! (79 seconds). - _Jason Earls_
%o (PARI) isok(n) = isprime(eval(concat(apply(x->Str(x), vector(n+1, j, if (j>n, n, 1)))))) \\ _Michel Marcus_, Jun 15 2018
%Y Cf. A068817, A084428.
%K base,nonn,more
%O 1,2
%A _Robert G. Wilson v_, May 03 2002
%E a(4) from _Jason Earls_, May 06 2002
%E a(5) from _Rick L. Shepherd_, Sep 02 2002
%E a(6) from _Farideh Firoozbakht_, Jun 29 2003
%E a(7) from _Farideh Firoozbakht_, Jul 04 2003
%E Edited by _T. D. Noe_, Oct 30 2008
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