Lark, thank you very much for your reply!

Unfortunately this does not work either. I did simply change my keyboard settings to english and now I can perform the subscript operation.

Hi, my programme works perfectly now,

Thanks a lot for your help!

Thank you so much, you guys are quick!

But now, anther prob occurred:

Previously I calculated the v-vector by using with(linalg): muliply(matrix1,vector1), and the output of that "multiplication" is NOT a vector, but an array or table. so I can not use D(v), because i get an error.

I did not find a multiply function in the VectorCalculus package. How can I do such a "multiplication" that puts out a vector?