12345...54321 (17350-digits)

This number is a probable-prime (likely to be a prime but we have not (re-)proven it on this site).

The second and largest known probable prime (1234567..244524462445..7654321) such that the decimal concatenation of the numbers ascend from 1 up through n followed by descending numbers from n−1 down through 1, for n=2446. [Gupta]