Many times you want to write the formulae in which the limits need to be written up side and low side of the sigma. But the problem happens when it is not written as expected.
In such times, just add \displaystyle
brfore the sum:
The problem:
$$\varrho(\tau) = \frac{ \sum_{t=1}^{T-\tau} (y_t - \mu) (y_{t + \tau} - \mu)} {(T - \tau)\varrho^2}$$
The solution