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 A246804 Numbers k such that (10^(k+2) + 999) * 10^k + 1 is prime. 1
 1, 3, 15, 135, 645, 1373, 195317, 237249 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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. LINKS MAPLE A246804:=n->`if`(isprime((10^(n+2)+999)*10^n+1), n, NULL): seq(A246804(n), n=1..10^3); # Wesley Ivan Hurt, Nov 16 2014 MATHEMATICA Select[Range[10^3], PrimeQ[(10^(# + 2) + 999)*10^# + 1] &] PROG (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)]; CROSSREFS Cf. A156166 (Belphegor's primes), A082703 (plateau primes 199...991). Sequence in context: A108210 A006717 A222263 * A230166 A059861 A232699 Adjacent sequences:  A246801 A246802 A246803 * A246805 A246806 A246807 KEYWORD nonn,more,base,hard AUTHOR Serge Batalov, Nov 16 2014 STATUS approved

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Last modified April 16 18:53 EDT 2021. Contains 343050 sequences. (Running on oeis4.)