Hi, I'm starting to study how to use Hubbard operators and I cannot understand one property:
Consider the hopping terms for a lattice Hamiltonian with bosons:
$$\sum_{i,j\neq i} t_{i,j} b^\dagger_i b_j$$
when writing this term in the basis of Hubbard operators $$X^{a,b}_i =| a,i \rangle \langle...