Im reading about fourier series, I have the solutions manual for a problem. But i dont understand it
The problem is
$ U(t) = \begin{cases} 1, & \text{0≤t<1} \\[2ex] 0, & \text{1≤t<2} \end{cases}$ $ U(t) = U(t+n*2)$
https://i.stack.imgur.com/n737S.png
The period is 2, so we have the lower limit 0 and upper limit 2. Why do the upper limits change?