i'm trying to solve system of 9 coupled linear differential equations.I've this problem ... what is it's meaning? enter image description here

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    $\begingroup$ Go ahead and post the code that leads to the error. $\endgroup$
    – Jason B.
    Sep 8, 2015 at 11:25
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    – rhermans
    Sep 8, 2015 at 11:34
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    $\begingroup$ Potential duplicates: Set::write posts -- Sorry no time to sift them....Ahmed, have you seen them? $\endgroup$
    – Michael E2
    Sep 8, 2015 at 12:47
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    $\begingroup$ You probably used a = somewhere where a == would have been appropriate. $\endgroup$ Sep 8, 2015 at 14:44
  • $\begingroup$ More potential duplicates: error deqn posts; in particular, (40314). Hard to tell, with the question in its current incomplete state. Have you searched for these things? Do any solve your problem? $\endgroup$
    – Michael E2
    Sep 8, 2015 at 14:52

1 Answer 1


You are likely using = (Set) instead of == (Equal) in the equations you are feeding to DSolve. You need to use ==, which is an equality test. Instead, you're trying to assign the right-hand side (presumably zero) to the linear combinations of As reported by Set::write. These linear combinations have the overall head Plus, which means that you're ultimately telling Mathematica something like


Here Plus is Protected, which means that Set can't assign it to anything - hence the error as reported.


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