# Riemannian metric question

#### Poirot

##### Banned
I'm interested in part iv) on the attachment. This is my work so far:
e=(1,0) and e'=(0,1) form a basis of the tangent space at any point z=(x,y). Making the identification (x,y)->x+iy, we get g(e,e')=0 and g(e,e)=g(e',e')=$\frac{1}{im(z)^2}$.

a(t)=z+t and b(t)=z+it are generating curves for e,e' respectively.
(lets call the function f)

$f(z)=\frac{z}{|z|^2}$ so $f(a(t))=\frac{z+t}{|z+t|^2}$. I need to find f'(a(t)) to proceed. How can I cope with differentiating the modulus of a complex number z? Thanks

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#### ZaidAlyafey

##### Well-known member
MHB Math Helper
Re: riemannian metric question

I don't know about metric spaces , but I know about complex analysis ... To differentiate a function a necessary requirement is to satisfy the cauchy-riemann equation .. suppose that $$\displaystyle f(z)=|z|$$ this function is clearly not differentiable

$$\displaystyle f(z)=\sqrt{x^2+y^2}$$

By the cauchy-reimann equation we must have $$\displaystyle \frac{\partial u}{\partial x}=\frac{\partial v}{\partial y}$$

Which is clearly not satisfied for $$\displaystyle |z|$$

The function you are trying to differentiate seems a function of several variables ? , are you differentiating with respect to t ?

#### Poirot

##### Banned
Re: riemannian metric question

I need to find the differential of f at z evaluated at e (and e'). This is equal to f'(a(t)) evaluated at t=0.