Transforming a matrix into a system of equations

asked 2017-04-04 05:21:45 +0100

Anonymous

I have a $n \times n$ matrix that I would like to transform into a system of $n$ equations (for $n$ disctinct variables). I am not sure how to proceed (the end goal being to solve the system eventually). The number of variables is not fixed. Should I consider every row of the lattice and map each coefficient $c_{ij}$ to a string that I write as $"c_{11}x_1 + .....+ c_{1n-1}x_n + c_{1n}$. I am hoping there exists a more direct solution. Thanks!

answered 2017-04-04 09:10:50 +0100

nbruin
4143 ●3 ●45 ●90

Something like:

sage: M=matrix(3,3,[1,2,3,4,5,6,7,8,9])
sage: P=PolynomialRing(QQ,M.nrows(),names="x")
sage: (vector(P.gen(i) for i in range(3))*M).list()
[x0 + 4*x1 + 7*x2, 2*x0 + 5*x1 + 8*x2, 3*x0 + 6*x1 + 9*x2]

perhaps? It will depend on your application a bit whether you need to convert to/from lists and what kind of polynomial ring you need. A good rule of thumb is avoiding going through strings. It will be much more robust if the system knows the meaning of the objects throughout.

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thank you ! I have two additional questions. It seems that your solution gives the column coefficient to a polynomial (and not the rows). Would it be possible to have the row coefficients? One way would be to simple transpose the matrix beforehand. I have tried using M.ncolumns() but it seems to generate an error. Also, is there anyway the last coefficient could be a constant variable (1) (in order to have $ax+by+c$ for example) ?

Sasha-dpt ( 2017-04-04 10:58:58 +0100 )edit

If you multiply on the other side: M*(vector(P.gen(i) for i in range(3))) sage considers the vector as a column rather than a row. The right length is M.ncols() in that case. You can build your vector any way you like. You can just append a "1": vector([P.gen(i) for i in range(2)] + [1])

nbruin ( 2017-04-04 17:43:02 +0100 )edit

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