A089922 Largest odd number in the reverse concatenation of the first n consecutive odd numbers when that concatenation is prime.

3, 73, 123, 817, 5433

Offset: 1

Author

Cino Hilliard, Jan 11 2004

Keywords

• nonn
• base
• more
• hard

Comments

These numbers are rare. For the case of the reverse concatenation of the consecutive numbers 1,2,3,... I have only found 828180...54321. Certainly, we can reduce the trials by noting that 2/3 of the numbers formed this way are multiples of 3 for the consecutive numbers and 1/3 of the odd consecutive numbers is a multiple of 3 but I do not think that will help much using PARI.

The number corresponding to 54335431...7531 has 10313 digits. The next term, if it exists is greater than 13000. - Dmitry Kamenetsky, Feb 21 2009

Examples

			31 is the reverse concatenation of the consecutive odd numbers 1 and 3, and it is prime.
817815813...531 is the reverse concatenation of the odd numbers 1 to 817, and it is a probable prime.
		

Crossrefs

Cf. original concatenation in A038395. - Dmitry Kamenetsky, Feb 21 2009

Programs

PARI

revprime2(n) = { y=1; forstep(x=3,n,2, y=concat(Str(x),Str(y)); z=eval(y); if(ispseudoprime(z),print(x, ",")) ) }

Extensions

Edited by T. D. Noe, Oct 30 2008

Added a new term 5433. This number is a probable prime with 20 iterations of Miller-Rabin test. - Dmitry Kamenetsky, Feb 21 2009