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A005939 Pseudoprimes to base 10.
(Formerly M4612)
13
9, 33, 91, 99, 259, 451, 481, 561, 657, 703, 909, 1233, 1729, 2409, 2821, 2981, 3333, 3367, 4141, 4187, 4521, 5461, 6533, 6541, 6601, 7107, 7471, 7777, 8149, 8401, 8911, 10001, 11111, 11169, 11649, 12403, 12801, 13833, 13981, 14701, 14817, 14911, 15211 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
This sequence is a subsequence of A121014 & A121912. In fact the terms are composite terms n of these sequences such that gcd(n,10)=1. Theorem: If both numbers q & 2q-1 are primes(q is in the sequence A005382) and n=q*(2q-1) then 10^(n-1) == 1 (mod n) (n is in the sequence A005939) iff mod(q, 20) is in the set {1, 7, 19}. 91,703,12403,38503,79003,188191,269011,... are such terms. - Farideh Firoozbakht, Sep 15 2006
Composite numbers n such that 10^(n-1) == 1 (mod n). - Michel Lagneau, Feb 18 2012
Composite numbers n such that the number of digits of the period of 1/n divides n-1. A number is pseudoprime to base 10 if the number of digits of the period of ((n-1)!+1)/n divides n-1. - Davide Rotondo, Dec 16 2020
REFERENCES
R. K. Guy, Unsolved Problems in Number Theory, A12.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
C. Pomerance & N. J. A. Sloane, Correspondence, 1991
MATHEMATICA
Select[Range[15300], ! PrimeQ[ # ] && PowerMod[10, (# - 1), # ] == 1 &] (* Farideh Firoozbakht, Sep 15 2006 *)
CROSSREFS
Cf. A001567 (pseudoprimes to base 2), A005382, A121014, A121912.
Sequence in context: A146171 A146188 A020228 * A020326 A201024 A228170
KEYWORD
nonn
AUTHOR
STATUS
approved

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Last modified June 18 08:27 EDT 2024. Contains 373472 sequences. (Running on oeis4.)