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 A109137 Numbers k such that k * (10^k - 1) + 1 is prime. 0
 2, 14, 42, 144, 200, 302, 7242, 8718, 10568, 24438 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Larger values certified with ECM. No more up to 6000. LINKS Table of n, a(n) for n=1..10. PROG (PARI) is(n)=ispseudoprime(n*(10^n-1)+1) \\ Charles R Greathouse IV, Jun 13 2017 (Python) from sympy import isprime def afind(limit, startk=1): k, pow10 = startk, 10**startk for k in range(startk, limit+1): if isprime(k*(pow10 - 1) + 1): print(k, end=", ") k += 1 pow10 *= 10 afind(500) # Michael S. Branicky, Aug 26 2021 CROSSREFS Cf. A110807. Sequence in context: A231081 A364218 A152670 * A192375 A267247 A297425 Adjacent sequences: A109134 A109135 A109136 * A109138 A109139 A109140 KEYWORD more,nonn AUTHOR Jason Earls, Aug 18 2005 EXTENSIONS a(7)-a(8) from Ryan Propper, Sep 20 2006 a(9) from Michael S. Branicky, Aug 26 2021 a(10) from Michael S. Branicky, Apr 05 2023 STATUS approved

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Last modified May 22 05:21 EDT 2024. Contains 372742 sequences. (Running on oeis4.)