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A089675 Numbers k such that 10^k - 3 is prime. 18
1, 2, 3, 17, 140, 990, 1887, 3530, 5996, 13820, 21873, 26045, 87720, 232599, 480684, 538640 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Numbers k such that 9*R_k - 2 is a prime number, where R_k = 11...1 is the repunit (A002275) of length k.
If k is in the sequence (10^k-3 is prime) and m=3*(10^k-3) then phi(m)=reversal(m), i.e., m is in the sequence A069215. - Farideh Firoozbakht, Dec 25 2004
No further terms for k <= 407197, see Kamada link.
LINKS
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^k).
MATHEMATICA
m = 1000; For[n = 1, n < m, If[PrimeQ[10^n - 3], Print[n]]; n++]
CROSSREFS
Sequence in context: A259535 A328340 A042978 * A041383 A042903 A367686
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
a(15) from Paul Bourdelais, Jan 06 2021
a(16) from Paul Bourdelais, Jan 28 2021
STATUS
approved

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Last modified March 29 02:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)