Sierpiński number
In number theory, a Sierpiński number is an odd natural number k such that is composite for all natural numbers n. In 1960, Wacław Sierpiński proved that there are infinitely many odd integers k which have this property.
In other words, when k is a Sierpiński number, all members of the following set are composite:
If the form is instead, then k is a Riesel number.
Known Sierpiński numbers
The sequence of currently known Sierpiński numbers begins with:The number 78557 was proved to be a Sierpiński number by John Selfridge in 1962, who showed that all numbers of the form have a factor in the covering set. For another known Sierpiński number, 271129, the covering set is. Most currently known Sierpiński numbers possess similar covering sets.
However, in 1995 A. S. Izotov showed that some fourth powers could be proved to be Sierpiński numbers without establishing a covering set for all values of n. His proof depends on the aurifeuillean factorization. This establishes that all give rise to a composite, and so it remains to eliminate only using a covering set.
Sierpiński problem
The Sierpiński problem asks for the value of the smallest Sierpiński number. In private correspondence with Paul Erdős, Selfridge conjectured that 78,557 was the smallest Sierpiński number. No smaller Sierpiński numbers have been discovered, and it is now believed that 78,557 is the smallest number.To show that 78,557 really is the smallest Sierpiński number, one must show that all the odd numbers smaller than 78,557 are not Sierpiński numbers. That is, for every odd k below 78,557, there needs to exist a positive integer n such that is prime. The distributed volunteer computing project PrimeGrid is attempting to eliminate all the remaining values of k:
The current status for the remaining multipliers can be seen at PrimeGrid's website.