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A247182
Numbers k such that the smallest k-digit odd number concatenated with the largest k-digit odd number is prime.
1
1, 3, 9, 21, 5571, 6493, 14481
OFFSET
1,2
COMMENTS
Numbers k such that 10^(k-1)+1 concatenated with 10^k-1 is prime.
a(7) > 10^4.
From Robert Israel, Jan 08 2017: (Start)
Numbers k such that 10^(2k-1) + 2*10^k - 1 is prime.
All terms are odd. (End)
a(8) > 24292. - J.W.L. (Jan) Eerland, Apr 08 2023
EXAMPLE
The smallest and largest odd 3-digit numbers are 101 and 999, respectively. Since 101999 is prime, 3 is a term 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 term 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
MATHEMATICA
k=1; Monitor[Parallelize[While[True, If[FromDigits[Join@@IntegerDigits/@{10^(k-1)+1, 10^k-1}]//PrimeQ, Print[k]]; k++]; k], k] (* J.W.L. (Jan) Eerland, Apr 08 2023 *)
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: A362341 A098980 A007647 * A363967 A318807 A063586
KEYWORD
nonn,hard,more,base
AUTHOR
Derek Orr, Nov 22 2014
EXTENSIONS
a(7) from J.W.L. (Jan) Eerland, Apr 08 2023
STATUS
approved