Suppose we are given a linear equation $ Ax=b $, where $ A \in Z_q^{n \times m} $ and $ b \in Z_q^n $.

Note that $ q $ is a prime here, and $ Rank(A)= Rank(A;b)=n<m $.

I wonder whether the following ROUCHÉ–CAPELLI THEOREM still holds in the finite field $ Z_q $:

Given a system of linear equations, $ Ax = b $, it holds that:

  1. $ Rank(A) = Rank(A; b) $ ⇔ the system is unsolvable.
  2. $ Rank(A) = Rank(A; b) $ ⇔ the system is solvable.
  3. $ Rank(A) = Rank(A; b) = m $ ⇔ the system has unique solution.
  4. $ Rank(A) = Rank(A; b) < m $ ⇔ the system has infinite solutions.

Is there any standard theorem in the thesis or textbook if it does? I need to cite the standard theorem into my thesis, but I can't find any.


  • 3
    $\begingroup$ How could you have an infinite number of solution when you only have a finite number of vectors of dimension $m$? Anyway, by the very definition of the rank (on any vector space, thus on vector space over $Z_q$ too), Rank(A)=Rank(A;b) exactly says that $b$ is linearly dependent on the column of $A$. Now, if you have a solution $x_0$, your set of solution is in one to one correspondence with $x_0+Ker(A)$, so you can deduce the number of solution from there. $\endgroup$
    – holf
    Oct 31, 2022 at 15:05
  • 3
    $\begingroup$ This looks like a question about pure math, with no computer science knowledge needed to answer it. As such it does not seem on-topic here. $\endgroup$
    – D.W.
    Oct 31, 2022 at 21:34


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