Perturbation theory of non-conjugate operators
USSR Computational Mathematics and Mathematical Physics
Volume 6, Issue 1, 1966, Pages 73-85
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B75BX-49090FJ-6R&_user=1002874&_coverDate=12%2F31%2F1966&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_acct=C000050161&_version=1&_urlVersion=0&_userid=1002874&md5=c3adb9bb050be8a0ebb7317bfa9242f0&searchtype=a