# Using CoefficientArrays to Invert simultaneous equations

Posted 4 months ago
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 The table obtained via the CoefficientArrays command of a list of vector equations does not seem to result in 3x3 Matrix as the coefficients of the original simultaneous equations.I tried using LinearSolve to invert the simultaneous equations, and I faced the same issue. (Deformation Equations) X = {X1, X2, X3}; x = {x1, x2, x3}; Eulerian = {e^-\[ScriptT]*x1 + 0*x2 + (e^-\[ScriptT] - 1)*x3, 0*x1 + x2 + (e^-\[ScriptT] - e^\[ScriptT])*x3, 0*x1 + 0*x2 + x3} CoefficientArrays[Eulerian, {x1, x2, x3}] Normal[%] // MatrixForm Take[%, {2}] // MatrixForm q = Flatten[%, 1] (* q is the Coefficients Matrix of x *) q // MatrixForm qInv = Inverse[q] // MatrixForm (* Eulerian in terms of q *) x = qInv . X(* Lagrangian Form *) (* Velocity Vector Field*) v = D[x, \[ScriptT]] // MatrixForm(* Material derivative*) 
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Posted 4 months ago
 A couple things: 1) When you do this: m = {{1,2},{3,4}}//MatrixForm m is no longer a matrix, it is a "Form" and m.{x,y} will not operate as you might expect. 2) It is much easier to debug if you put your input into separate Input cells and evaluate one at a time.I've rewritten things here: X = {X1, X2, X3}; x = {x1, x2, x3}; Eulerian = {e^-\[ScriptT]*x1 + 0*x2 + (e^-\[ScriptT] - 1)*x3, 0*x1 + x2 + (e^-\[ScriptT] - e^\[ScriptT])*x3, 0*x1 + 0*x2 + x3} I suspect you want Exp[..] or E^(..) instead of e? It is a good habit to define your own symbols with lower case letters. ca = CoefficientArrays[Eulerian, {x1, x2, x3}] (*same as CoefficientArrays[Eulerian, x]*) (nca = Normal[ca]) // MatrixForm notice that the = is done before the output is turned into a "Form"Also, you can just do this directly: Inverse[ca[[2]]] Continuing.... (tnca = Take[nca, {2}]) // MatrixForm (*do you mean nca[[2]] here? tnca has dimensions {1,3,3}, and nca[[2]] has dimensions {3,3} I'll keep working as though Take is what you want*) q = Flatten[tnca, 1] q has dimensions {3,3} now qInv = Inverse[q] You are redefining x here. x = qInv . X And this gives an expression: D[x,-\[ScriptT]] 
Posted 3 months ago
 Thanks, Craig Very helpful, and it fixed my code!
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