‘He used what he called the ‘fluxio-differential’ method, so called because it used two independent first-order Leibniz type differential operators.’

‘In these papers a special role is played by expansions in terms of the eigenfunctions of linear differential operators.’

‘He showed how to extend Laplace's series to give a perturbing function in the case of elliptical orbits by introducing differential operators which act on the Laplace coefficients.’

‘Formal properties of differential operators are studied in many of his contributions, in particular in his early papers he worked on commuting differential operators.’

‘And we all know that the Laplacian operator is by far the most important differential operator in Mathematical Physics.’

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