General Chen's first inequality and applications for Riemannian maps

In this paper, we propose \textit{general Chen's first inequality} for Riemannian maps between Riemannian manifolds and manifest its equality and sharpness via non-trivial examples. We also utilize this general inequality by establishing Chen's first inequalities when the target spaces are generalized complex and generalized Sasakian space forms, including real, complex, real K\"ahler, Sasakian, Kenmotsu, cosymplectic, and almost $C(\alpha)$ space forms. In addition, we estimate $\delta$-invariants under all possible hypotheses on these space forms. Finally, we validate our new approach by comparing particular results with those of existing approaches.