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A247183
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Numbers n such that the largest n-digit odd number concatenated with the smallest n-digit odd number is prime.
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1
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OFFSET
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1,1
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COMMENTS
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Numbers n such that 10^n-1 concatenated with 10^(n-1)+1 is prime.
a(8) > 10^4.
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LINKS
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EXAMPLE
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The largest and smallest odd 3-digit numbers are 999 and 101, respectively. Since 999101 is prime, 3 is a member of the sequence.
The largest and smallest odd 4-digit numbers are 9999 and 1001, respectively. Since 99991001 is not prime, 4 is not a member of this sequence.
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PROG
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(PARI) f(n) = 10^(n-1)*(10^(n+1)-9)+1
for(n=1, 10^4, if(ispseudoprime(f(n)), print1(n, ", ")))
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CROSSREFS
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KEYWORD
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nonn,base,more,hard
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AUTHOR
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STATUS
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approved
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