A b s t r a c t . The problem of the existence of an asymptotic expansion of type f (x) = anx n + an−1x n−1 + · · · + aix i + o(x i ), x → +∞, is thoroughly studied, comparing and completing the known results obtained through the three different approaches mentioned in the title. A unifying thread is provided by the canonical factorizations of the differential operator D n . Particularly meaningful are several characterizations of the polynomial asymptotic expansions of an nth order convex function.