INTEGRAL REPRESENTATIONS FOR BRACKET-GENERATING MULTI-FLOWS

If f(1), f2 are smooth vector fields on an open subset of an Euclidean space and [f(1), f(2)] is their Lie bracket, the asymptotic formula Psi([f1,f2]) (t(1), t(2))(x)-x = t(1)t(2) [f(1), f(2)](x)+ o(t(1)t(2)), where we have set Psi[f(1),f(2)] (t(1,)t(2))(x) -(def) exp(-t(2)f(2)) o exp(-t(1)f(1)) o exp(t(2)f(2)) o exp(t(1)f(1))(x), is valid for all t(1), t(2) small enough. In fact, the integral, exact formula Psi([f1,f2]) (t(1), t(2))(x)-x =integral(t1)(0)integral(t2)(0) [f(1),f(2])((s2,s1))(Psi(t(1),s(2))(x))ds(1) ds(2), where [f(1), f(2)]((s2,s1))(y)=(def) D (exp(s(1)f(1))o exp(s(2)f(2))))(-1)(y).[f(1), f(2)](exp(s(1)f(1)) o exp(s(2)f(2))(y)), has also been proven. Of course (2) can be regarded as an improvement of (1). In this paper we show that an integral representation like (2) holds true for any iterated Lie bracket made of elements of a family f(1),...., f(m) of vector fields. In perspective, these integral representations might lie at the basis for extensions of asymptotic formulas involving non-smooth vector fields.