Polynomial bound on the distribution of poles in scattering by an obstacle
Journées équations aux dérivées partielles, (1984), p. 1-8 
@article{JEDP_1984____A3_0,
     author = {Melrose, Richard B.},
     title = {Polynomial bound on the distribution of poles in scattering by an obstacle},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     publisher = {Ecole polytechnique},
     year = {1984},
     pages = {1-8},
     zbl = {0621.35073},
     language = {en},
     url = {http://www.numdam.org/item/JEDP_1984____A3_0}
}

Melrose, Richard B. Polynomial bound on the distribution of poles in scattering by an obstacle. Journées équations aux dérivées partielles,  (1984), pp. 1-8. https://www.numdam.org/item/JEDP_1984____A3_0/

[1] P.D. Lax & R.S. Phillips. Scattering theory, Academic Press. | Zbl 0117.09104

[2] R.B. Melrose. Polynomial bound for the poles in scattering by a potential. J. Funct. Anal (1984).