@article{IM_1982__69_142953,
     author = {J.J. Duistermaat and G.J. Heckman},
     title = {On the {Variation} in the {Cohomology} of the {Symplectic} {Form} of the {Reduced} {Phase} {Space.}},
     journal = {Inventiones mathematicae},
     pages = {259--268},
     publisher = {mathdoc},
     volume = {69},
     year = {1982},
     zbl = {0503.58015},
     url = {http://geodesic.mathdoc.fr/item/IM_1982__69_142953/}
}