@inproceedings{m.1973lacunas,
title={Lacunas for hyperbolic differential operators with constant coefficients. II},
author={M. F. Atiyah, R. Bott, and L. Gårding},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203229949370156},
booktitle={Acta Mathematica},
volume={131},
number={1},
pages={145-206},
year={1973},
}
M. F. Atiyah, R. Bott, and L. Gårding. Lacunas for hyperbolic differential operators with constant coefficients. II. 1973. Vol. 131. In Acta Mathematica. pp.145-206. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203229949370156.