83893153674119714347

This number is a prime.

                                                       8389315367 4119714347

The largest 20-digit prime that remains prime by deleting from the right every time a double-digit prime, i.e., 838931536741197143, 8389315367411971, 83893153674119, 838931536741, 8389315367, 83893153, 838931, 8389, 83. [Loungrides]