Past exam of the mathematics course of the University of Cambridge / 2018 / ii / Paper 1 / 26I / d / Solution

The answer is no. Let $a_i\ne 0$ tend to zero and takeAt a point $x\in X_a$ with $a\ne 0$, differentiating the defining equations givesThusfor every $i$, and $w_i\to w=(0,1,0,0)$. But $X_0$ is the $x_4$-axis, soHenceThe obstruction is the dimension jump in a smooth family of manifolds.