proof of rank-nullity theorem


Let T:VW be a linear mapping, with V finite-dimensionalPlanetmathPlanetmath. We wish to show that

dimV=dimKerT+dimImgT

The images of a basis of V will span ImgT, and hence ImgT is finite-dimensional. Choose then a basis w1,,wn of ImgT and choose preimages v1,,vnU such that

wi=T(vi),i=1n

Choose a basis u1,,uk of KerT. The result will follow once we show that u1,,uk,v1,,vn is a basis of V.

Let vV be given. Since T(v)ImgT, by definition, we can choose scalars b1,,bn such that

T(v)=b1w1+bnwn.

Linearity of T now implies that T(b1v1++bnvn-v)=0, and hence we can choose scalars a1,,ak such that

b1v1++bnvn-v=a1u1+akuk.

Therefore u1,,uk,v1,,vn span V.

Next, let a1,,ak,b1,,bn be scalars such that

a1u1++akuk+b1v1++bnvn=0.

By applying T to both sides of this equation it follows that

b1w1++bnwn=0,

and since w1,,wn are linearly independentMathworldPlanetmath that

b1=b2==bn=0.

Consequently

a1u1++akuk=0

as well, and since u1,,uk are also assumed to be linearly independent we conclude that

a1=a2==ak=0

also. Therefore u1,,uk,v1,,vn are linearly independent, and are therefore a basis. Q.E.D.

Title proof of rank-nullity theorem
Canonical name ProofOfRanknullityTheorem
Date of creation 2013-03-22 12:25:13
Last modified on 2013-03-22 12:25:13
Owner rmilson (146)
Last modified by rmilson (146)
Numerical id 4
Author rmilson (146)
Entry type Proof
Classification msc 15A03