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A331815
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Numbers k such that 10^(2*k) - 8*10^(k-1) - 1 is prime.
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0
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OFFSET
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1,1
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COMMENTS
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Also numbers k such that the concatenation (k 9's)1(k-1 9's) is prime.
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LINKS
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EXAMPLE
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3 is a term because 999199 is prime.
4 is a term because 99991999 is prime.
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MATHEMATICA
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Select[Range[500], PrimeQ[10^(2*#) - 8*10^(#-1) - 1] &] (* Amiram Eldar, Jan 28 2020 *)
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PROG
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(PARI) (is_A331815(n)=ispseudoprime(100^n-8*10^(n-1)-1)); for(n=1, 9999, is_A331815(n)&&print1(n", "))
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CROSSREFS
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Cf. A077776 = A183184*2+1: palindromic near-repdigit primes 9..919..9.
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KEYWORD
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nonn,base,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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