Determining Newforms via various relations among Fourier Coefficients

In this article, we investigate how arithmetic relations among the Fourier coefficients of two newforms can be used to determine the underlying forms. For pairs of twist-inequivalent non-CM newforms, we obtain quantitative results on the distribution of differences, products, and ratios of normalized Fourier coefficients at prime powers. As applications, we derive quantitative refinements of multiplicity one and establish density one analogues of a theorem of Gafni–Thorner–Wong. We further show that, for twist-inequivalent newforms, the ratios of Fourier coefficients are quantitatively dense in the real line, which provides a new criterion for determining newforms through the distribution of their coefficient ratios.