such that lim(n→∝)sup x_n < r. Show that there is an integer N∈N
such that x_n<r whenever n ≥ N.
更新1:
不好意思有符號顯現不出來 such that lim(n→∝)sup x_n 小於 r. Show that ...
更新2:
蜉蝣大大好，我們課本上用的符號意思有點不一樣 s_n = sup(k≥n) x_k = sup{x_k: k≥n}
