Let $V$ be a variety over a field $F$. A point $P$ of $V$ is **non-singular** if the module of differentials of $V$ is locally free at $P.$ According to the Jacobian criterion, if $V$ is defined in a neighborhood of $P$ by affine polynomial equations $f_1(X_1, \ldots, X_n) = \ldots =f_r(X_1, \ldots, X_n)=0,$ then $V$ is non-singular at $P$ if the Jacobian matrix $\left( \frac{\partial f_i}{\partial X_j} \right)_{ij}$ has the same rank as the codimension of $V$ in $\mathbb A^n.$