Definition A function f between vector spaces V and W over a field F is a linear map iff f(λ1⋅v1+Vλ2⋅v2)=λ1⋅(f v1)+Wλ2⋅(f v2)(∀v1,v2∈V;λ1,λ2∈F) References pinter2010-abstract-algebra (ch28)