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A246804
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Numbers k such that (10^(k+2) + 999) * 10^k + 1 is prime.
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1
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OFFSET
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1,2
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COMMENTS
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Or, indices of primes in the sequence of decimal palindromes 19991, 1099901, 100999001, 10009990001, ...
Or, numbers k such that there exists an "upside-down-Belphegor's primes" of length 2*k+3.
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LINKS
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MAPLE
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MATHEMATICA
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Select[Range[10^3], PrimeQ[(10^(# + 2) + 999)*10^# + 1] &]
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PROG
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(PARI) for( n=1, 9999, ispseudoprime((10^(n+2)+999)*10^n+1) & print1(n", "))
(Magma) [n: n in [1..500] | IsPrime((10^(n+2)+999)*10^n+1)];
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CROSSREFS
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KEYWORD
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nonn,more,base,hard
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AUTHOR
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STATUS
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approved
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