@article{CMH_1974__49_139572,
     author = {Martin Schechter},
     title = {Scattering {Theory} for {Elliptic} {Operators} of {Arbitrary} {Order}},
     journal = {Commentarii mathematici Helvetici},
     pages = {84--113},
     publisher = {mathdoc},
     volume = {49},
     year = {1974},
     zbl = {0288.47012},
     url = {http://geodesic.mathdoc.fr/item/CMH_1974__49_139572/}
}