Mean value theorems for the double zeta-function
  We prove asymptotic formulas for mean square values of the Euler double zeta-function $\zeta_2(s_0,s)$, with respect to $\Im s$. Those formulas enable us to propose a double analogue of the Lindel{\"o}f hypothesis. 
Kohji Matsumoto, Hirofumi Tsumura
math.NT