login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

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.

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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

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