# If $f^2$ is an analytic function then so is $f$

I want to show the following:

If $f(z)$ is a continuous function on a connected open subset of the complex plane and $f(z)^2$ is an analytic function, then $f(z)$ is analytic.

Clearly if $f(z) \neq 0$ then $$\frac{f(z+h)-f(z)}{h}=\frac{f(z+h)^2-f(z)^2}{h}.\frac{1}{f(z+h)+f(z)}$$

which shows $f^\prime(z)$ exists if $f(z)\neq0$.

What do I do when $f(z) =0$?

• Do you know about analytic continuation? Applying it should help you... Feb 18, 2013 at 13:30

## 2 Answers

a) Since $f^2$ is holomorphic, the Cauchy-Riemann criterion (expressed in Wirtinger's wonderfully concise notation) $\partial f^2/\partial \bar z=2f\partial f/\partial \bar z=0$, shows that $f$ is holomorphic at all points $z$ where $f(z)\neq 0$, since there $\partial f/\partial \bar z=0$. [This is the part solved by Don Antonio in more classical notation]

b) Since $f^2$ is holomorphic its zeros are isolated and so are those of $f$ (they are the same!)
But $f$ is locally bounded at those potential singularities, because $f^2$ is, and so Riemann's theorem on removable singularities permits you to conclude that the singularity is bogus and that actually $f$ is holomorphic also at the zeros of $f$ .

Conclusion: $f$ is holomorphic everywhere on its domain.

• so in your b> part : zeros of $f^2$ are same as the zeros of $f$ and zeros of $f$ are isolated (as zeros of $f^2$ are isolated ). so $f$ is analytic possibly at these zeros and then use riemanns thrm
– jim
Feb 18, 2013 at 14:09
• Dear jim: yes, precisely. Feb 18, 2013 at 14:12
• sorry for not accepting your answer i upvoted it thanks for the explanation
– jim
Feb 18, 2013 at 14:13
• +1 for using the derivative wrt $\,z\,$ notation, which I didn't want to use because I thought it'd make things harder to understand to the OP, but no doubt renders a neater proof. Feb 18, 2013 at 14:26
• Dear DonAntonio, I am sure you knew Wirtinger's notation. The decision to use more advanced notation/concepts is always a very problematic one and your choice is very reasonable. I find it optimal that readers now may select themselves the version they are more comfortable with. Feb 18, 2013 at 14:50

Suppose

$$f(x,y)=u(x,y)+iv(x,y)=:u+iv\Longrightarrow f^2=u^2-v^2+2uvi$$

Since $$\,f^2\,$$ is analytic then the Cauchy-Riemann equations apply here:

$$(u^2-v^2)'_x=(2uv)'_y\;\;\;,\;\;\;(u^2-v^2)'_y=-(2uv)'_x$$

But

$$(u^2-v^2)'_x=2uu_x-2vv_x\;\;\,\;\;(2uv)'_y=2u_yv+2uv_y\\(u^2-v^2)'_y=2uu_y-2vv_y\;\;,\;\;-(2uv)'_x=-2u_xv-2uv_x$$

So equalling:

$$I\;\;\;\;uu_x-vv_x=\;\;\;\;uv_y+vu_y\Longleftrightarrow\;\, u(u_x-v_y)-v(u_y+v_x)=0\\II\;\;\;\;uu_y-vv_y\;=-uv_x-vu_x\Longleftrightarrow u(u_y+v_x)+v(u_x-v_y)=0$$

Can you take it now from here (i.e., check the CR equations for $$\,f=u+iv\,$$) ?

• if $u$ and $v$ are both not $0$ then $Ax=0$ has nonzero solution which implies $\det(A) =0\implies (u_x-v_y)^2+(u_y+v_x)^2=0$ from this cr equation follows. here $A= \begin{bmatrix} u_x-v_y & u_y+v_x \\ u_y+v_x & u_x-v_y \\ \end{bmatrix}$
– jim
Feb 18, 2013 at 13:45
• The only delicate point is precisely to prove analyticity at the points where $u=v=0$, i.e. at the points where $f$ vanishes. [And that is the reason why I decided to answer this question: in order that users have a possible solution also for that case on record. I also wanted to advertise the power of Wirtinger's notation] Anyway, +1 for DonAntonio. Feb 18, 2013 at 13:56
• Hi, I also have the same question so don't want to ask it again. Wondering how you "take it from here now". I dont get any cancellation from adding or subtracting the two equations so how can I deduce the terms in the parentheses are zero? Apr 18, 2018 at 20:37
• I think.. this is wrong answer because we don't know $u, v$ are partially differentiable fucntion. we just know that $f^2$ is analytic
– hew
Jul 19, 2019 at 7:55
• @hew You seem to be missing the point: never in the answer I did assume $\;f\;$ is analytic: I only used the given data that $\;f^2\;$ is analytic and thus the CR equations apply to it...and from this I deduce equations from which we can have the CR equations for $\;f\;$ . Jul 19, 2019 at 8:30