Definition: Cartesian multiplication
Cartesian multiplication of two sets A, B with |a>,|b> elements is defined as
Definition: Projection
Projection f from set A to set B is AxB subset defined as

and notation
is used.
Definition: Body of numbers
Body of numbers T is defined as
Definition: Vector space
Vector space V is defined as
Definition: Function
Function is a projection
, which meets
Definition: Functional
Functional is a projection
, which meets
Definition: Operator
Operator is a projection
, which meets