# Problem 62: Principal Component Analysis with Nonnegativity Constraints

Suggested by | Andrea Montanari |
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Source | Bertinoro 2014 |

Short link | https://sublinear.info/62 |

Given a symmetric matrix $A$, we can think of Principal Component Analysis (PCA) as maximizing $x^\top A x$ subject to $\|x\|=1$. If we also add the condition $x \ge 0$, this problem becomes NP-hard. We can define a convex relaxation:

\[ \max \operatorname{Tr}(WA) \quad \text{s.t.} \quad \operatorname{Tr}(W) = 1,\ \ W \ge 0,\ \ W \succeq 0. \]

Suppose that $A$ is a random matrix. In particular, set $A_{ij}$ to be i.i.d $N(0,1)$. Then empirical results show that the resulting $W$ is a rank-1 matrix, which means that we recover the optimal $x$ exactly.

Is this true in general? Note that we can prove that the solution is rank 1 if $A = v v^T + (\text{small amount of noise})$.