I have to determine the $Matrix F$ which describes f in comparison to the base A(a1,a2,a3,a4),
and the matrix B - which describes regarding the basis E=(e1,e2,e3,e4) for $R^4$
That is
$F =a[f]a$
$B=e[f]e$
From a previous calculation, i know that:
$f(x)=(x \cdot a1)a2+(x \cdot a2)a3 +(x \cdot a3)a4$
Where $a1,a2...$ is vectors forming an orthogonal basis $R^4$
But I dont get the meaning of F=a[f]a