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

1,1

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

Links

Table of n, a(n) for n=1..5.

Example

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.

Prog

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

Crossrefs

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

Sequence in context: A139888 A271628 A079594 * A259848 A041279 A054699

Adjacent sequences: A089919 A089920 A089921 * A089923 A089924 A089925

Keyword

nonn,base,more,hard

Author

Cino Hilliard, Jan 11 2004

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

Status

approved