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A089675
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Numbers k such that 10^k - 3 is prime.
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18
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1, 2, 3, 17, 140, 990, 1887, 3530, 5996, 13820, 21873, 26045, 87720, 232599, 480684, 538640
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OFFSET
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1,2
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COMMENTS
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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.
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LINKS
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FORMULA
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EXAMPLE
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10^2 - 3 = 97 is a prime number (in fact all terms are the largest less than 10^k).
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MATHEMATICA
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m = 1000; For[n = 1, n < m, If[PrimeQ[10^n - 3], Print[n]]; n++]
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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Michael Gottlieb (mzrg(AT)verizon.net), Jan 05 2004
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EXTENSIONS
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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
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STATUS
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approved
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