@article{IM_1975__29_142329,
     author = {J.J. Duistermaat and V.W. Guillemin},
     title = {The {Spectrum} of {Positive} {Elliptic} {Operators} and {Periodic} {Bicharacteristics.}},
     journal = {Inventiones mathematicae},
     pages = {39--80},
     publisher = {mathdoc},
     volume = {29},
     year = {1975},
     zbl = {0307.35071},
     url = {http://geodesic.mathdoc.fr/item/IM_1975__29_142329/}
}