<正> Cauchy问题的唯一性是偏微分方程的基本问题之一.经典的Cauchy-Kowalewski定理断言,解析方程或方程组的解析解是唯一的.1901年,Holmgren证明了,线性的解析方程或方程组的光滑解的唯一性.在取消关于系数的解析性的假设这个方向上的第一个结果是由Carleman在1939年给出的,他证明了两个自变量的相应结果,其中假设方程的主部的系数是实的,以及特征根是单重的,因而特征根的虚部如果不恒为零则总不为零.

The uniqueness in Cauchy problems for linear partial differential equations with C~∞ coefficients is of quite complication. In 1974, M. Strauss and F. Treves proved uniqueness in the Cauchy problem u_t+ib(x, t)u_x+c(x,t)u=0 and u(x, 0)=0, where t b(0,t) vanishes of finite order at t=0. In this paper, by using the technique of pseudo-differential operators, we give a general result on the uniqueness in Cauchy problems for systems of equations of order m≥1 with two independent variables.