This paper concens about a Carleman estimate which can give the unique continuation property of solutions for a multi-terms time fractional diffusion equation up to order α(0<α<2) with general time dependent second order strongly elliptic operator for the diffusion. By using a special Holmgren type transformation which is linear with respect to time, the estimate giving a local unique continuation of solutions is derived via some subelliptic estimate for an operator associated to this transformed equation using calculus of pseudo-differential operators. After that we have given a new argument to derive the global unique continuation of solutions. Here the global unique continuation means as follows. If u is a solution of the multi-terms time fractional diffusion equation in a domain over the time interval (0, T) and it is supported on t≥ 0 , then a zero set of solution over a subdomain of Ω can be continued to (0 , T) × Ω.
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