In the study of mathematics and especially differential geometry, fundamental vector fields are an instrument that describes the infinitesimal behaviour of a smooth Lie group action on a smooth manifold. Such vector fields find important applications in the study of Lie theory, symplectic geometry, and the study of Hamiltonian group actions. Motivation Important to applications in mathematics and physics is the notion of a flow on a manifold. In particular, if is a smooth manifold and is a smooth vector field, one is interested in finding integral curves to . More precisely, given one is interested in curves such that for which local solutions are guaranteed by the Existence and Uniqueness Theorem of Ordinary Differential Equations. If is furthermore a complete vector field, then the flow of , defined as the collection of all integral curves for , is a diffeomorphism of . The flow given by is in fact an action of the additive Lie group on . Conversely, every smooth action defines a complete vector field via the equation It is then a simple result that there is a bijective correspondence between actions on and complete vector fields on . In the language of flow theory, the vector field is called the infinitesimal generator. Intuitively, the behaviour of the flow at each point corresponds to the "direction" indicated by the vector field. It is a natural question to ask whether one may establish a similar correspondence between vector fields and more arbitrary Lie group actions on . Definition Let be a Lie group with corresponding Lie algebra . Furthermore, let be a smooth manifold endowed with a smooth action . Denote the map such that , called the orbit map of corresponding to . For , the fundamental vector field corresponding to is any of the following equivalent definitions: where is the differential of a smooth map and is the zero vector in the vector space . The map can then be shown to be a Lie algebra homomorphism. Applications Lie groups The Lie algebra of a Lie group may be identified with either the left- or right-invariant vector fields on . It is a well known result that such vector fields are isomorphic to , the tangent space at identity. In fact, if we let act on itself via right-multiplication, the corresponding fundamental vector fields are precisely the left-invariant vector fields. Hamiltonian group actions In the motivation,