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 A057175 Numbers n such that (9^n + 1)/10 is a prime. 18
 3, 59, 223, 547, 773, 1009, 1823, 3803, 49223, 193247, 703393, 860029 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Repunit primes in base -9. - Paul Bourdelais LINKS P. Bourdelais,A Generalized Repunit Conjecture H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7. H. Lifchitz, Mersenne and Fermat primes field Eric Weisstein's World of Mathematics, Repunit MATHEMATICA lst={}; Do[p=(9^n+1)/10; If[PrimeQ[p], AppendTo[lst, n]], {n, 7!}]; lst (* Vladimir Joseph Stephan Orlovsky, Sep 29 2008 *) Select[Range[4000], PrimeQ[(9^# + 1)/10] &] (* Vincenzo Librandi, Aug 03 2015 *) PROG (PFGW v3.3.1 from primeform.net) pfgw -b2 -f10 bNeg9.txt:: ABC2 (9^\$a+1)/10 // -f{4*\$a} a: primes from 3 to 1e6} (MAGMA) [n: n in [0..800] | IsPrime((9^n + 1) div 10 )]; // Vincenzo Librandi, Aug 03 2015 (PARI) first(m)=my(v=vector(m)); t=0; for(i=1, m, while(!isprime((9^t + 1)\10), t++); v[i]=t; t++; ); v; \\ Anders HellstrÃ¶m, Aug 16 2015 CROSSREFS Sequence in context: A002148 A290977 A200957 * A142642 A201850 A013526 Adjacent sequences:  A057172 A057173 A057174 * A057176 A057177 A057178 KEYWORD nonn AUTHOR N. J. A. Sloane, Sep 15 2000 EXTENSIONS a(9) corresponds to a probable prime, was discovered on Oct 22 2007. Trial factored to 1E11 with Fermat base 2 primality test. - Paul Bourdelais a(10) corresponds to a probable prime, discovered by Paul Bourdelais, Feb 01 2010 a(11) corresponds to a probable prime, discovered by Paul Bourdelais, Aug 03 2015 a(12) corresponds to a probable prime, discovered by Paul Bourdelais, Sep 23 2020 STATUS approved

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Last modified October 21 05:38 EDT 2020. Contains 337911 sequences. (Running on oeis4.)