# Completeness relation

wasi-uz-zaman
hi, in quantum physics completeness relation is often use it equals to one - what is its significance in multiplying with state vector . thanks
wasi

$$\sum {| n \rangle \langle n |} = 1\\ \sum {| n \rangle \langle n | \psi \rangle} = | \psi \rangle$$
$$\psi(x) = \sum {c_n \psi_n(x)}$$