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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
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.
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
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