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A005422 Largest prime factor of 10^n - 1.
(Formerly M2889)
7
3, 11, 37, 101, 271, 37, 4649, 137, 333667, 9091, 513239, 9901, 265371653, 909091, 2906161, 5882353, 5363222357, 333667, 1111111111111111111, 27961, 10838689, 513239, 11111111111111111111111, 99990001, 182521213001, 1058313049 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

REFERENCES

J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Ray Chandler, Table of n, a(n) for n = 1..322 (first 100 terms from T. D. Noe)

K. Beschorner, Factorizations of Repunit Numbers of the form (10^n-1)/9 (111111...). [Retrieved July 30, 2015]

J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.

Yousuke Koide, Factorizations of Repunit Numbers.

S. S. Wagstaff, Jr., The Cunningham Project

FORMULA

For n > 1, a(n) = A003020(n). For 1 < n < 10, a(n) = A075024(n). - M. F. Hasler, Jul 30 2015

a(n) = A006530(A002283(n)). - Vincenzo Librandi, Jul 13 2016

MAPLE

A005422 := proc(n)

    10^n-1 ;

    A006530(%) ;

end proc: # R. J. Mathar, Dec 02 2016

MATHEMATICA

Table[FactorInteger[10^n - 1][[-1, 1]], {n, 1, 40}] (* Vincenzo Librandi, Jul 13 2016 *)

PROG

(PARI) a(n)=vecmax(factor(10^n-1)[, 1]) \\ Simplified by M. F. Hasler, Jul 30 2015

(MAGMA) [Maximum(PrimeDivisors(10^n-1)): n in [1..45]]; // Vincenzo Librandi, Jul 13 2016

CROSSREFS

Same as A003020 except for the additional a(1) = 3.

Cf. A002283, A006530, A067063, A075024, A102380.

Cf. similar sequences listed in A274906.

Sequence in context: A046107 A243110 A061075 * A040017 A007615 A065540

Adjacent sequences:  A005419 A005420 A005421 * A005423 A005424 A005425

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

EXTENSIONS

Edited by M. F. Hasler, Jul 30 2015

STATUS

approved

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Last modified June 1 12:29 EDT 2020. Contains 334762 sequences. (Running on oeis4.)