equation expression problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg101431] equation expression problem*From*: Haibo Min <haibo.min at gmail.com>*Date*: Tue, 7 Jul 2009 05:06:14 -0400 (EDT)

Hi, everyone. I am now conducting a simulation for spacecraft attitude control. In this problem, q denotes the attitude which is expressed in quaternion, i.e., q={Vq,Sq}, where Vq is a vector consisting of the first three elements of q, and Sq is a scalar which is the last element of q, so Vq=q[[1;;3]], Sq=q[[4]]. The dynamics of the spacecraft consist of Vq , Sq, and q. What I want to do is to express the dynamics by q alone, therefore, in constructing the equation table, I used the following code: eqns[n_Integer] /; Positive[n] := Table[{ Subscript[q, i][[1;;3]]'[t]== "expression a ", Subscript[q, i][[4]]'[t]=="expression b", "expression c"== " an expression including Subscript[q, i][t]" }, {i,n}] but error occurs, Part::take:cannot take positions 1 through 3 in q1. part::partw:part of q1 does not exist... It seems that mathematica does not know what q is and therefore can not take its elements. I know that I can change it into another expression like Subscript[Vq, i]'[t] == "expression a " Subscript[Sq, i]'[t] == "expression b " Subscript[q, i][t] == Append[Subscript[Vq, i]'[t], Subscript[Sq, i][t]]. ... In another word, denote the dynamics by Vq and Sq as well as q respectively, but it is not convenient. Is there any suggestion? Thanks in advance. Best regards, Haibo

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