[paideic]

Presumably multiplying by two and adding one? That's the pattern that jumped out to me, unless they're talking about something more subtle.

[oisdk]

Oh, of course! Looking at it now that's pretty obvious, lol.

Although I think even if I had spotted that I still would have put it into oeis to see if there was a quick membership test.

[mankeysee]

Which is to say, 2^n-1.