A Note on the Asymptotic Expansion of Matrix Coefficients over $p$-adic Fields

In this note, presented as a ``community service", followed by the PhD research of the author, we draw the relation between Casselman’s theorem regarding the asymptotic behavior of matrix coefficients of reductive algebraic groups over $p$-adic fields and its expression as a finite sum of finite functions. In addition, we write the expansion explicitly for general linear groups.