TeX source:

\sigma_{t}^{2}=\alpha_{0}+\alpha_{1} \varepsilon_{t}^{2}+\ldots+\alpha_{q} \varepsilon_{t-q}^{2}+\beta_{1} \sigma_{t-1}^{2}+\ldots+\beta_{p} \sigma_{t-p}^{2}=\alpha_{0}+\sum_{i=1}^{q} \alpha_{i} \varepsilon_{t-i}^{2}+\sum_{i=1}^{p} \beta_{i} \sigma_{t-i}^{2}