Let be a real linear space. The mapping is called a "scalar product" if it has the following properties:
1. ,
2. The mapping is linear for each ,
3.
4. .
The real linear space equipped with a scalar product is called "Hilbert space" if it is a Banach space with respect to the norm .
(Riesz representation theorem). For each there exists a such that
(Lax-Milgram theorem). Let be a Hilbert space and be a bilinear mapping: for some constants . Fix an . There exists a unique such that