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Return a linear operator's diagonal elements.
openfermion.linalg.get_linear_qubit_operator_diagonal( qubit_operator, n_qubits=None )
The main motivation is to use it for Davidson's algorithm, to find out the lowest n eigenvalues and associated eigenvectors.
Qubit terms with X or Y operators will contribute nothing to the diagonal elements, while I or Z will contribute a factor of 1 or -1 together with the coefficient.
A qubit operator.
The diagonal elements for LinearQubitOperator(qubit_operator).