Symbolic Math Error using diff

Question

Carlas 2013-11-3

编辑： Carlas 2013-11-3

Hi All,

I was wondering why it is not possible to do the following

x = sym('x',[2 1]); A = sym('A',[2 2]);

diff(x'*A*x,'x')

ans = 0

Is there an alternative method to preform this differentiation?

Thanks

Carlas 2013-11-3

编辑：Carlas 2013-11-3

gradient(x'*A*x,x) gives the requested result ans =

 A1_1*x1 + A1_2*x2 + A1_1*conj(x1) + A2_1*conj(x2)
 A2_1*x1 + A2_2*x2 + A1_2*conj(x1) + A2_2*conj(x2)

Does anyone know a way to get the result in matrix form again: (A+A')x

Answer 1

sixwwwwww 2013-11-3

Dear Carlas, you can try something like this:

x = sym('x%d',[2 1]); 
A = sym('A%d%d',[2 2]);
x1 = diff(x' * A * x, x(1));
x2 = diff(x' * A * x, x(2));

I hope it helps. Good luck!