Transcendental entire solutions of several general quadratic type PDEs and PDDEs in ${\mathbb{C}}^2$

The functional equations $f^2+g^2=1$ and $f^2+2\alpha fg+g^2=1$ are respectively called Fermat-type binomial and trinomial equations. It is of interest to know about the existence and form of the solutions of general quadratic functional equations. Utilizing Nevanlinna’s theory for several complex variables, in this paper, we study the existence and form of the solutions to the general quadratic partial differential or partial differential-difference equations of the form $a{f}^2+2\alpha fg+b{g}^2+2\beta f+2\gamma g+C=0$ in ${\mathbb{C}}^2$. Consequently, we obtain certain corollaries of the main results of this paper concerning binomial equations which generalize many results in [$\mathit{\text{Rocky}}\mathit{\text{Mountain}}\mathit{\text{J.}}\mathit{\text{Math.}}$ $\mathbf{\text{51}}$(6) (2021), 2217-2235] in the sense of arbitrary coefficients.