42000...00043 (19750-digits)

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

The largest known prime consisting of 42, then a run of zeros, then 43: 42*10^19748 + 43, with 19,746 zeros and 19,750 digits. It is the 15th known member of the family 42*10^(k+2) + 43 (k = 0, 1, 3, 6, 21, 40, 81, 108, 383, 417, 1067, 1686, 4506, 7842, 19746); no other member occurs for k up to 33900 (search checked Oct 2026). Every member has digit sum 13 and is never divisible by 3, 7 or 11, and for each of twelve other small primes exactly one residue class of k is excluded — e.g., k congruent to 2 mod 6 makes the number divisible by 13. [Bean]