Construction, Extension and Coupling of Frames on Finite Dimensional   Pontryagin Space
  In this paper we extend to finite-dimensional Pontryagin spaces the methods used in \cite{CasazzaLeon,Deguang} to build frames from an adjoint and positive operator. It is proved that any frame in finite dimensional Pontryagin space $\mathcal{K}$ is $J$-orthogonal projection of a frame for a space $\mathcal{H}$ such that $\mathcal{K}\subset\mathcal{H}$. Furthermore, given $\{k_{n}\}_{n=1}^{m}$ and $\{x_{n}\}_{n=1}^{k}$ frames for $\mathcal{K}$ and $\mathcal{H}$ respectively, we build a finite-dimensional Pontryagin space $\mathfrak{K}$ and a frame $\{y_{n}\}_{n=1}^{N}$ for $\mathfrak{K}$ such that $\mathcal{K},\mathcal{H}\subset\mathfrak{K}$ and $\{k_{n}\}_{n=1}^{m},\{x_{n}\}_{n=1}^{k}\subset \{y_{n}\}_{n=1}^{N}$. 
German Escobar, Kevin Esmeral, Osmin Ferrer
math.FA