Low-rank tensor approximations are plagued by a well-known problem - a tensor may fail to have a best rank-$r$ approximation. Over $\mathbb{R}$, it is known that such failures can occur with positive probability, sometimes with certainty. We will show that while such failures still occur over $\mathbb{C}$, they happen with zero probability. In fact we establish a more general result with useful implications on recent scientific and engineering applications that rely on sparse and/or low-rank approximations: Let $V$ be a complex vector space with a Hermitian inner product, and $X$ be a closed irreducible complex analytic variety in $V$. Given any complex analytic subvariety $Z \subseteq X$ with $\dim Z < \dim X$, we prove that a general $p \in V$ has a unique best $X$-approximation $\pi_X (p)$ that does not lie in $Z$. In particular, it implies that over $\mathbb{C}$, any tensor almost always has a unique best rank-$r$ approximation when $r$ is less than the generic rank. Our result 查看全文>>