Method diagonal

ibtk/contrib/eigen/Eigen/src/SparseCore/SparseMatrix.h:655–655

\returns a const expression of the diagonal coefficients. */

    /** \returns a const expression of the diagonal coefficients. */
    const ConstDiagonalReturnType diagonal() const { return ConstDiagonalReturnType(*this); }
    
    /** \returns a read-write expression of the diagonal coefficients.
      * \warning If the diagonal entries are written, then all diagonal