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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

Table of n, a(n) for n=1..6.

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.)