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 A004022 Primes of the form (10^n - 1)/9. (Formerly M4816) 91
 11, 1111111111111111111, 11111111111111111111111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term corresponds to n = 317 and is too large to include: see A004023. Also called repunit primes or prime repunits. Also, primes with digital product = 1. The number of 1's in these repunits must also be prime. Since the number of 1's in (10^n-1)/9 is n, if n = pk then (10^pk-1)=(10^p)^k-1 => (10^p-1)/9 = q and q divides (10^n-1). This follows from the identity a^n - b^n = (a-b)(a^(n-1) + a^(n-2)b + ... + b^(n-1)). - Cino Hilliard, Dec 23 2008 A subset of A020449, ..., A020457, A036953, ..., cf. link to OEIS index. - M. F. Hasler, Jul 27 2015 The terms in this sequence, except 11 which is not Brazilian, are prime repunits in base ten, so they are Brazilian primes belonging to A085104 and A285017. - Bernard Schott, Apr 08 2017 REFERENCES T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, p. 11. Graham, Knuth and Patashnik, Concrete mathematics, Addison-Wesley, 1994; see p. 146, problem 22. Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). M. Barsanti, R. Dvornicich, M. Forti, T. Franzoni, M. Gobbino, S. Mortola, L. Pernazza and R. Romito, Il Fibonacci N. 8 (included in Il Fibonacci, Unione Matematica Italiana, 2011), 2004, Problem 8.10. LINKS T. D. Noe, Table of n, a(n) for n = 1..5 J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002. Makoto Kamada, Factorizations of 11...11 (Repunit). D. H. Lehmer, On the number (10^23-1)/9, Bull. Amer. Math. Soc. 35 (1929), 349-350. Andy Steward, Prime Generalized Repunits S. S. Wagstaff, Jr., The Cunningham Project FORMULA a(n) = A002275(A004023(n)). MATHEMATICA lst={}; Do[If[PrimeQ[p = (10^n - 1)/9], AppendTo[lst, p]], {n, 10^2}]; lst (* Vladimir Joseph Stephan Orlovsky, Aug 22 2008 *) Select[Table[(10^n - 1) / 9, {n, 500}], PrimeQ] (* Vincenzo Librandi, Nov 08 2014 *) Select[Table[FromDigits[PadRight[{}, n, 1]], {n, 30}], PrimeQ] (* Harvey P. Dale, Apr 07 2018 *) PROG (PARI) forprime(x=2, 20000, if(ispseudoprime((10^x-1)/9), print1((10^x-1)/9", "))) \\ Cino Hilliard, Dec 23 2008 (MAGMA) [a: n in [0..300] | IsPrime(a) where a is (10^n - 1) div 9 ]; // Vincenzo Librandi, Nov 08 2014 CROSSREFS See A004023 for the number of 1's. Cf. A046413. Sequence in context: A257166 A257167 * A243534 A257304 A241570 A083344 Adjacent sequences:  A004019 A004020 A004021 * A004023 A004024 A004025 KEYWORD nonn,nice,bref AUTHOR EXTENSIONS Edited by Max Alekseyev, Nov 15 2010 STATUS approved

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Last modified September 22 18:50 EDT 2018. Contains 315270 sequences. (Running on oeis4.)