  Now symbolical algebra came into play by simply treating the operator \(D^2 - D - 2\) as though it were an ordinary polynomial in algebra. Boole’s 1841 paper On the integration of linear differential equations with constant coefficients gave a nice improvement to Gregory’s method for solving such differential equations, an improvement based on a standard tool in algebra, partial fractions,  which he applied to the reciprocal of differential operators like the above.