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 A247182 Numbers n such that the smallest n-digit odd number concatenated with the largest n-digit odd number is prime. 1
 1, 3, 9, 21, 5571, 6493 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that 10^(n-1)+1 concatenated with 10^n-1 is prime. a(7) > 10^4. From Robert Israel, Jan 08 2017: (Start) Numbers n such that 10^(2n-1) + 2*10^n - 1 is prime. All terms are odd. (End) LINKS EXAMPLE The smallest and largest odd 3-digit numbers are 101 and 999, respectively. Since 101999 is prime, 3 is a member of the sequence. The smallest and largest odd 4-digit numbers are 1001 and 9999, respectively. Since 10019999 is not prime, 4 is not a member of this sequence. MAPLE select(n -> isprime(10^(2*n-1)+2*10^n-1), [seq(i, i=1..1000, 2)]); # Robert Israel, Jan 08 2017 PROG (PARI) f(n)=eval(concat(Str(10^(n-1)+1), 10^n-1)) for(n=1, 10^4, if(ispseudoprime(f(n)), print1(n, ", "))) CROSSREFS Cf. A247183. Sequence in context: A191998 A098980 A007647 * A318807 A063586 A131477 Adjacent sequences:  A247179 A247180 A247181 * A247183 A247184 A247185 KEYWORD nonn,hard,more,base AUTHOR Derek Orr, Nov 22 2014 STATUS approved

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Last modified June 26 13:53 EDT 2022. Contains 354883 sequences. (Running on oeis4.)