@inproceedings{c.1960commutators,,
title={Commutators, perturbations, and unitary spectra},
author={C. R. Putnam},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203157924513928},
booktitle={Acta Mathematica},
volume={106},
number={3},
pages={215-232},
year={1960},
}