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 A056703 Numbers n such that 3*10^n - 1 is prime. 1
 0, 1, 3, 6, 7, 19, 27, 43, 55, 207, 1311, 3204, 7050, 9439, 26044, 33058, 34507, 49314, 119292 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that 2*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. LINKS Table of n, a(n) for n=1..19. Makoto Kamada, Prime numbers of the form 299...99. Index entries for primes involving repunits MATHEMATICA Do[ If[ PrimeQ[ 2*10^n + (10^n-1)], Print[n]], {n, 0, 3000}] Select[Range[200000], PrimeQ[3*10^#-1]&] (* Harvey P. Dale, Sep 29 2012 *) PROG (PARI) is(n)=ispseudoprime(3*10^n-1) \\ Charles R Greathouse IV, Jun 12 2017 CROSSREFS Sequence in context: A333553 A341183 A259256 * A113534 A103831 A217519 Adjacent sequences: A056700 A056701 A056702 * A056704 A056705 A056706 KEYWORD hard,nonn,more AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 STATUS approved

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Last modified June 21 07:48 EDT 2024. Contains 373541 sequences. (Running on oeis4.)