Hans Havermann recently mentioned Leyland primes (primes in the form x^y + y^x), so I looked them up on OEIS. I expected to see a bunch of random-looking odd numbers. And for the most part, that's what I saw. But I noticed that some of them have remarkably long runs of 0s. For instance the 30th term is a 465-digit decimal number, of which 305 consecutive digits are 0. And the 37th term is a 613-digit number of which 417 consecutive digits are 0. Why should that be? And does something similar happen in other radices? Thanks. One possibility is that in those cases y^x is much greater than x^y and y is divisible by 10. If so, I'd expect that I'd find primes in the form x^y - y^x to often have long runs of 9s. I found a list at A123206, and see that that is indeed the case. So I guess that's the explanation.