# Sum of outer product of vectors in a basis

If $\{|u_1\rangle, ..., |u_n\rangle \}$ are an orthonormal basis for $\mathbb{C}_n$, then

$$\sum_{j=1}^{n} |u_j\rangle\langle u_j| = I_n$$

I can see that this is true in the standard computational basis, but I'm having trouble seeing it intuitively when generalized to any basis, nor can I prove it. Can anyone help?

Let $A$ be the operator give by your sum.
• Can you see that $A | u_i \rangle = | u_i \rangle$ for each $i$?
• Can you see why the above observation implies that $A$ is the identity?