The number of solutions to a linear system

Created almost 9 years ago, updated about 8 hours ago

Over $\mathbb R$ or $\mathbb C$, a linear system has zero, one, or infinitely many solutions. An inconsistent system has none. For a consistent rank-$r$ system with $n$ unknowns, the solution set is $x_p+\ker A$ and has $n-r$ free parameters.

If $n-r=0$, the solution is unique. If it is positive, the infinite field supplies infinitely many distinct parameter choices. Equivalently, two distinct solutions generate an entire affine line of solutions.

Over a finite field $\mathbb F_q$, the corresponding number is $q^{n-r}$ when consistent. Thus the infinite-solution classification must not be transferred unchanged to coding theory.

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