A Shanks 
-chain
is a sequence of primes 
of the form
 |
with 
and 
integers. On Sep. 1, 2000, P. Leyland found a (4,
17)-chain of length 6, and on Nov. 22, 2000, he reported the first known chain
of length 7. Its first member is 176996212597 and the seventh has 758 digits.
