Elementary first integrals of differential equations
M. J. Prelle, Michael F. Singer · Transactions of the American Mathematical Society · 1983
We show that if a system of differential equations has an elementary first integral (i.e. a first integral expressible in terms of exponentials, logarithms and algebraic functions) then it must have a first integral of a very simple form. This unifies and extends results of Mordukhai-Boltovski, Ritt and others and leads to a partial algorithm for finding such integrals.