

A121014


Nonprime terms in A121912.


4



1, 6, 9, 10, 15, 18, 30, 33, 45, 55, 90, 91, 99, 165, 246, 259, 370, 385, 451, 481, 495, 505, 561, 657, 703, 715, 909, 1035, 1045, 1105, 1233, 1626, 1729, 2035, 2409, 2465, 2821, 2981, 3333, 3367, 3585, 4005, 4141, 4187, 4521, 4545, 5005, 5461, 6533, 6541
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OFFSET

1,2


COMMENTS

Theorem: If both numbers q and 2q1 are primes(q is in the sequence A005382) and n=q*(2q1) then 10^n == 10 (mod n) (n is in the sequence A121014) iff q<5 or mod(q, 20) is in the set {1, 7, 19}. 6,15,91,703,12403,38503,79003,188191,269011,... are such terms. A005939 is a subsequence of this sequence.  Farideh Firoozbakht, Sep 15 2006


LINKS

Table of n, a(n) for n=1..50.


FORMULA

Theorem: If both numbers q and 2q1 are primes and n=q*(2q1) then 10^n == 10 (mod n) (n is in the sequence) iff q<5 or mod(q, 20) is in the set {1, 7, 19}.  Farideh Firoozbakht, Sep 11 2006


MATHEMATICA

Select[Range[10^4], ! PrimeQ[ # ] && PowerMod[10, #, # ] == Mod[10, # ] &] (*Chandler*)


PROG

(PARI) for(n=1, 7000, if(!isprime(n), k=10^n; if((k10)%n==0, print1(n, ", "))))  (Klaus Brockhaus, Sep 06 2006)


CROSSREFS

Cf. A005382, A005939.
Sequence in context: A145311 A037198 A054020 * A153519 A020219 A175634
Adjacent sequences: A121011 A121012 A121013 * A121015 A121016 A121017


KEYWORD

nonn,easy


AUTHOR

N. J. A. Sloane, Sep 06 2006


EXTENSIONS

Extended by Ray Chandler and Klaus Brockhaus, Sep 06 2006


STATUS

approved



