A Colbert number is any prime number with more than  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.

There are currently five known Colbert numbers, as summarized in the following table.

Colbert number	decimal digits
	1521561
	3918990
	2759677
	2357207
	2116617

The Seventeen or Bust distributed computing effort is conducting a search for the remaining six Colbert numbers (where  indicates the exponent is unknown).

unknown Colbert number	decimal digits
	?
	?
	?
	?
	?
	?

REFERENCES:

Caldwell, C. "The Prime Glossary: Colbert Numbers". https://primes.utm.edu/glossary/xpage/ColbertNumber.html.

Helm, L.; Moore, P.; Samidoost, P.; and Woltman, G. "Resolution of the Mixed Sierpinski Problem." Integers: Elec. J. Combin. Numb. Th. 8, No. A61, 2008.

Helm, L. and Norris, D. "Seventeen or Bust: A Distributed Attack on the Sierpinski Problem." https://www.seventeenorbust.com/.