> 23 digits 91919191919191919191919 is prime

The 15 digit number 151515151515151 is prime, too ;

> 17 digits 91919191919191919 is prime.

> 11 digits 91919191919 is prime.

> 9 digits 919191919 is prime.

and it is the only such prime (undulating : aba...ba

with ab = 10*a+b or ba = 10*b+a digits.

M.