A247183 Numbers k such that the largest k-digit odd number concatenated with the smallest k-digit odd number is prime.

3, 7, 17, 51, 343, 423, 6453

Offset

1,1

Comments

Numbers k such that 10^k-1 concatenated with 10^(k-1)+1 is prime.

a(8) > 10^4.

a(8) > 5*10^4. - Michael S. Branicky, Dec 05 2024

All terms are odd. - Robert Israel, Dec 05 2024

Links

Table of n, a(n) for n=1..7.

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.

Maple

select(n -> isprime(10^(2*n)-9*10^(n-1)+1), [seq(i, i=3..6500, 2)]); # Robert Israel, Dec 05 2024

Prog

(PARI) f(n) = 10^(n-1)*(10^(n+1)-9)+(n > 1)
for(n=1, 10^4, if(ispseudoprime(f(n)), print1(n, ", ")))

Crossrefs

Cf. A247182.

Sequence in context: A090977 A324789 A014144 * A321139 A096358 A359174

Adjacent sequences: A247180 A247181 A247182 * A247184 A247185 A247186

Keyword

nonn,base,more,hard

Author

Derek Orr, Nov 22 2014

Status

approved