

A089675


Numbers n such that 9*R_n  2 is a prime number, where R_n = 11...1 is the repunit (A002275) of length n.


18



1, 2, 3, 17, 140, 990, 1887, 3530, 5996, 13820, 21873, 26045, 87720, 232599
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OFFSET

1,2


COMMENTS

Also numbers n such that 10^n  3 is prime.
If n is in the sequence (10^n3 is prime) and m=3*(10^n3) then phi(m)=reversal(m), i.e., m is in the sequence A069215.  Farideh Firoozbakht, Dec 25 2004
No further terms for n<=407197, see Kamada link.


LINKS

Table of n, a(n) for n=1..14.
Makoto Kamada, Prime numbers of the form 99...997.
Index entries for primes involving repunits.


FORMULA

a(n) = A056662(n) + 1


EXAMPLE

10^2  3 = 97 is a prime number (in fact all terms are the largest less than 10^n).


MATHEMATICA

m = 1000; For[n = 1, n < m, If[PrimeQ[10^n  3], Print[n]]; n++]


CROSSREFS

Cf. A002275, A056662, A069215, A093172, A101700.
Sequence in context: A164816 A259535 A042978 * A041383 A042903 A132534
Adjacent sequences: A089672 A089673 A089674 * A089676 A089677 A089678


KEYWORD

more,nonn


AUTHOR

Michael Gottlieb (mzrg(AT)verizon.net), Jan 05 2004


EXTENSIONS

a(8) from Robert G. Wilson v, Jan 14 2004
a(9) and a(10) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 06 2004
a(11) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 13 2004
a(12) from Henri Lifchitz.
Edited by Patrick De Geest, Dec 28 2004
Edited by Ray Chandler, Dec 23 2010


STATUS

approved



