Many zeta functions and L-functions which are generalizations of the Riemann zeta function play very important roles in the modern mathematics (Kummer criterion, class number formula, Weil conjecture, BSD conjecture, Langlands program, Riemann hypothesis,...).
Maybe, Euler iswas perhaps the first person whoto consider the zeta function $\zeta(s)$ ($1\leq s$). Why did Euler study such a function? What was thehis aim of Euler?
Further, though we know their importance well, we should thinkwe consider that the Riemann zeta functionsfunction and its generalizations happen to play key roles in the modern number theory?