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In multiples papers or online article on the QAOA algorithm (such as this one), I found notation for the Hamiltonian similar to this one :

$$ \sum_{ij} \frac{1}{2} (I-\sigma_i^z \sigma_j^z)$$

I don't know how to interpret the notation $\sigma_j^z$. I guess it has something to do with the Pauli matrix $\sigma_z$ but that's all I can understand.