A Colbert number is any prime number with more than 1,000,000 decimal digits whose discovery contributes
to the long-sought after proof that 
is the smallest Sierpiński
number of the second kind. Colbert numbers are named to honor Stephen T. Colbert.
Six Colbert numbers are known as of October 2026, as summarized in the following table.
| Colbert number | decimal digits |
 | 1521561 |
 | 9383761 |
 | 3918990 |
 | 2759677 |
 | 2357207 |
 | 2116617 |
The independent Seventeen or Bust project ceased operations in 2016, and the search continues through PrimeGrid's Seventeen or Bust subproject (PrimeGrid 2016). The
five remaining forms are listed below, where 
denotes an unknown exponent.
| unknown Colbert number | decimal digits |
 | ? |
 | ? |
 | ? |
 | ? |
 | ? |
