Linear Algebra – Transpose Matrix Dual Map

linear algebramatrices

How do I see that the representing matrix of the dual map $f^*$ between finite-dimensional dual spaces is given by the transpose of the representing matrix of $f$? Here I want to assume that the matrix $f^*$ is represented with respect to the dual basis.

Apparently this result is very well-known, but I would like to see proof of this.

Best Answer

Let $f: V \to \bar V$ be a linear transformation represented by the matrix $A = [a^i_j]$ relative to the basis $e_1,\dots,e_n$ for $V$ and $\bar e_1, \dots, \bar e_m$ for $\bar V$. Let $\alpha^1, \dots, \alpha^n$ and $\bar \alpha^1, \dots, \bar \alpha^m$ denote the respective corresponding dual bases.

Suppose that $f^*:\bar V^* \to V^*$ satisfies $f^*(\bar \alpha^i) = \sum_{j} b^i_j \alpha^j$. That is, suppose that $[b^i_j]^T$ is the matrix of $f^*$ relative to the dual bases. It follows that for each $i,j$, we have \begin{align*} b^i_j & = \sum_{k} b^i_k \delta^k_j = \sum_{k} b^i_k \alpha^k e_j = \left(\sum_{k} b^i_k \alpha^k\right) e_j = f^*(\bar \alpha^i) e_j = (f^* \circ \bar \alpha^i) e_j \\ & = (\bar \alpha^i \circ f)(e_j) = \bar \alpha^i(f(e_j)) = \bar \alpha^i\left(\sum_{k}a^k_j \bar e_k\right) = \sum_{k}a^k_j \bar \alpha^i \bar e_k = \sum_{k}a^k_j \delta^i_k = a^i_j \end{align*} as desired. That is, the matrix of $f^*$ with respect to the dual bases is $[b^i_j]^T = [a^i_j]^T = A^T$.