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 A247183 Numbers n such that the largest n-digit odd number concatenated with the smallest n-digit odd number is prime. 1
 3, 7, 17, 51, 343, 423, 6453 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that 10^n-1 concatenated with 10^(n-1)+1 is prime. a(8) > 10^4. LINKS EXAMPLE 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. PROG (PARI) f(n) = 10^(n-1)*(10^(n+1)-9)+1 for(n=1, 10^4, if(ispseudoprime(f(n)), print1(n, ", "))) CROSSREFS Cf. A247182. Sequence in context: A090977 A324789 A014144 * A321139 A096358 A260349 Adjacent sequences:  A247180 A247181 A247182 * A247184 A247185 A247186 KEYWORD nonn,base,more,hard AUTHOR Derek Orr, Nov 22 2014 STATUS approved

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Last modified June 29 15:47 EDT 2022. Contains 354913 sequences. (Running on oeis4.)