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In the array $a_n=\frac{7\times 10^n-1}{3}$, are there infinitely many primes?

(when $n={7+16k},a_n$ is divisible by $17$, so there are infinitely numbers not prime)

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    FYI: http://oeis.org/A198972 – mathlove Aug 07 '14 at 12:34
  • Related question: http://math.stackexchange.com/questions/34877/are-there-infinitely-many-primes-and-non-primes-of-the-form-10n1?rq=1. – Dietrich Burde Aug 07 '14 at 13:56
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    My hunch is that there are infinitely many such primes, though you might easily be able to rule out multiples of 19, 29, 31: see http://factordb.com/index.php?query=%287+*+10%5En+-+1%29%2F3&use=n&n=1&VP=on&VC=on&EV=on&OD=on&PR=on&FF=on&PRP=on&CF=on&U=on&C=on&perpage=200&format=1 –  Aug 07 '14 at 19:51

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