A version of the Davis–Kahan Tan 2 theorem  for not necessarily semibounded linear operators defined by quadratic forms is proven. This theorem generalizes a result by Motovilov and Selin .
Cite this article
Luka Grubišić, Vadim Kostrykin, Konstantin A. Makarov, Krešimir Veselić, The Tan 2 Theorem for indefinite quadratic forms. J. Spectr. Theory 3 (2013), no. 1, pp. 83–100