Linear mapping/Dual mapping/Functorial properties/Fact/Proof

Proof

(1). For , we have

(2) follows directly from .

(3). Let and

Because of the surjectivity of , there exist for every a such that . Therefore

and is itself the zero mapping. Due to fact, injective.

(4). The condition means that we may consider as a linear subspace. Because of fact, we can write

with another -linear subspace . A linear form

can always be extended to a linear form

for example, by defining on to be the zero form. This means the surjectivity.