# Find f(z) if possible from u(x,y) and v(x,y)

1. Nov 4, 2012

### IHazAName

1. The problem statement, all variables and given/known data

a. u(x,y)=(x+2)2-y2, v(x,y)=2(x+2)y
b. u(x,y)=cosxcoshy, v(x,y)=-sinxsinhy
c. u(x,y)=(x2+y2+y)/(x2+y2), v(x,y)=x/(x2+y2)
d. u(x,y)=eycosx, v(x,y)=eysinx

3. The attempt at a solution

a. f(x+iy)=(x+2)2-y2+i2(x+2)y
=(x+2+iy)(x+2+iy)
f(z)=(z+2)2

b. f(x-iy)=cosxcoshy-isinxsinhy
Am I allowed to answer this question as f($\bar{z}$)=cos$\bar{z}$?

c.f(x-iy)=((x2+y2+y)/(x2+y2))-i(x/(x2+y2))
=((x-iy-i)(x+iy))/((x-iy)(x+iy))
=(x-iy-i)/(x-iy)
Same as b... f($\bar{z}$)=($\bar{z}$-i)/$\bar{z}$?

d. du/dx != dv/dy, fails Cauchy-Reimann?

Edit: How do I go from x-iy to x+iy? This escapes me. :(

Last edited: Nov 4, 2012
2. Nov 5, 2012

### tiny-tim

Welcome to PF!

Hi IHazAName! Welcome to PF!

Is the question to find an f such that f = u + iv ?

a and b look ok

(for b, just replace y by -y )

I haven't checked c

For d, isn't cauchy-riemann only relevant to differentiability? Try again

3. Nov 5, 2012

### HallsofIvy

Staff Emeritus
If $f(\bar{z})= cos(\bar{z})$, then f(z)= cos(z).

And, as in b, if $f(\bar{z})= (\bar{z}- i)/\bar{z}$ then f(z)= (z- i)/z.

The problem asked you to find a function- it did not say the function had to be analytic.

If z= x- iy, then $\bar{z}= x+ iy$.

4. Nov 5, 2012

### IHazAName

Thanks guys!

...Oops. I forgot to mention that...these are supposed to be analytic functions as that is what we're learning. :(

So, just to make extra sure: for f(x-iy)=cos(x)cosh(y)-isin(x)sinh(y) that is still f(z)=cos(z)? Even though using x+iy grants f(x+iy)=cos(x)cosh(y)+isin(x)sinh(y)

I guess my confusion with that in our notes we only solve it one way...where we're given a f(z)...say f(z)=z and plug in x+iy so that f(z) becomes f(x+iy)=x+iy, and solve for u(x,y) and v(x,y). In these problems I was just trying to go in the reverse direction, so getting a result that needed x-iy was confusing to me.