# Math Help - bilinear forms

1. ## bilinear forms

the problem statement is, Prove that there is a unique bilinear form f, on Um X Vn such that, f(ui,vj)=aij where aij, i=1,...,m j=1,..n are given scalars, {u1,u2,...um} and {v1,v2,...vn} are ordered bases for Um and Vn respectively.

i've got how to prove it is bilinear but how do i prove uniqueness?

2. Originally Posted by neha18
the problem statement is, Prove that there is a unique bilinear form f, on Um X Vn such that, f(ui,vj)=aij where aij, i=1,...,m j=1,..n are given scalars, {u1,u2,...um} and {v1,v2,...vn} are ordered bases for Um and Vn respectively.

i've got how to prove it is bilinear but how do i prove uniqueness?
Just assume that you are given a second billinear form g that satisfies $g(u_i,v_j)=a_{ij}$ as well (and, therefore $g(\vec{u}_i,\vec{v}_j)=f(\vec{u}_i,\vec{v}_j)$ for all $i=1,\ldots,m; j=1,\ldots,n$) and then show that for all vectors $\vec{x}, \vec{y}$ it is true that $g(\vec{x},\vec{y})=f(\vec{x},\vec{y})$.
You do this by replacing $\vec{x}$ and $\vec{y}$ by linear combinations of the basis vectors, $\vec{x}=x_1\vec{u}_1+\cdots+x_m\vec{u}_m$ and $\vec{y}=y_1\vec{v}_2+\cdots+y_n\vec{v}_n$ for example, and apply bilinearity...