Inner product space
Any Euclidean space $ \mathbf{R}^{n} $ with dot product is an inner product space.
$ \left\langle \left(x_{1},\cdots,x_{n}\right),\left(y_{1},\cdots,y_{n}\right)\right\rangle \triangleq\sum_{i=1}^{n}x_{i}y_{i} $
