

A089922


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


3




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



EXAMPLE

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



KEYWORD

nonn,base


AUTHOR



EXTENSIONS

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


STATUS

approved



