Motivation
(f(mathbf{x}) = oldsymbol{eta}^ opmathbf{x})
[frac{df}{dmathbf{x}} = egin{bmatrix}
frac{df}{dx_1} \
vdots \
frac{df}{dx_n}
end{bmatrix} = egin{bmatrix}
eta_1 \
vdots \
eta_n
end{bmatrix} = oldsymbol{eta}]
(frac{d}{dmathbf{x}}(mathbf{x}^ op A mathbf{x}) = (mathbf{A} + mathbf{A}^ op)mathbf{x}.)