this up there is same as if you write
if there is , so that then we say that sequence converge.
if you look at sequence you know that it's not converge (it diverges) but let's try to show that
okay, let's assume that converges, and that there is for some we have that . if we look at members of the sequence we see that all of them can be (1) or (-1), that means that both of those numbers have to be in any region of the point a. but that is not possible if you chose that your there is no way to both of those numbers to be in that interval which length is less the one
"we say that sequence converges to the point if in every region of the point are almost all members of the sequence "