单位阶跃序列¶

\[ \varepsilon(k) := \left\{\begin{matrix} 0, &k<0 \\ 1, &k\ge0 \end{matrix}\right. \]

\[ \varepsilon(k)=\sum_{i=-\infty}^{k}\delta(i) \]

\[ \varepsilon(k)=\sum_{i=0}^{+\infty}\delta(k-i) \]