On the behavior of multiple zeta-functions with identical arguments on the real line
We study the behavior of $r$-fold zeta-functions of Euler-Zagier type with identical arguments $ζ_r(s,s,\ldots,s)$ on the real line. Our basic tool is an "infinite'' version of Newton's classical identities. We carry out numerical computations, and draw graphs of $ζ_r(s,s,\ldots,s)$ for real $s$, for several small values of $r$. Those graphs suggest various properties of $ζ_r(s,s,\ldots,s)$, some of which we prove rigorously. When $s \in [0,1]$, we show that $ζ_r(s,s,\ldots,s)$ has $r$ asymptotes at $\Re s=1/k$ ($1\leq k\leq r$), and determine the asymptotic behavior of $ζ_r(s,s,\ldots,s)$ close to those asymptotes. Numerical computations establish the existence of several real zeros for $2\leq r\leq 10$ (in which only the case $r=2$ was previously known). Based on those computations, we raise a conjecture on the number of zeros for general $r$, and gives a formula for calculating the number of zeros. We also consider the behavior of $ζ_r(s,s,\ldots,s)$ outside the interval $[0,1]$. We prove asymptotic formulas for $ζ_r(-k,-k,\ldots,-k)$, where $k$ takes odd positive integer values and tends to $+\infty$. Moreover, on the number of real zeros of $ζ_r(s,s,\ldots,s)$, we prove that there are exactly $(r-1)$ real zeros on the interrval $(-2n,-2(n-1))$ for any $n \geq 2$.