The TAN $2\mathrm{\Theta }$ Theorem for Indefinite Quadratic Forms

A version of the Davis-Kahan Tan $2\mathrm{\Theta }$ theorem [SIAM J. Numer. Anal. $\mathbf{\text{7}}$ (1970), 1 – 46] for not necessarily semibounded linear operators defined by quadratic forms is proven. This theorem generalizes a recent result by Motovilov and Selin [Integr. Equat. Oper. Theory $\mathbf{\text{56}}$ (2006), 511 – 542].