$f(z_0)=w_0$ Then $exists epsilon, delta$ such that $f(z)-alpha$ has exactly $m$ simple roots in $B(z_0,delta)$ for $|alpha-w_0|<epsilon$

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Let $f$ be analytic at $z_0 in mathbbC$ and $f(z_0)=w_0$. Suppose that $f(z)-w_0$ has a zero of finite order in $m geq 1$ at $z=z_0$.



We need to show that there exist $epsilon >0$ and $delta >0$ such that for all $alpha in mathbbC$ with $|alpha -w_0|<epsilon$, $f(z)-alpha$ has exactly $m$ simple roots in $B(z_0,delta)$.




Looking for some Hints. I wish to use to use $$frac12pi iint_mathcalCfracf'(z)f(z)dz = textnumber of zeros of f text in Int(mathcalC)$$.







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  • $alpha = w_0$??
    – mathworker21
    Aug 3 at 9:26










  • you still have $alpha = w_0$
    – mathworker21
    Aug 3 at 9:28










  • just corrected it
    – Arindam
    Aug 3 at 9:28










  • was my answer below helpful?
    – mathworker21
    2 days ago










  • Yes, thanks. It was helpful.
    – Arindam
    yesterday














up vote
0
down vote

favorite













Let $f$ be analytic at $z_0 in mathbbC$ and $f(z_0)=w_0$. Suppose that $f(z)-w_0$ has a zero of finite order in $m geq 1$ at $z=z_0$.



We need to show that there exist $epsilon >0$ and $delta >0$ such that for all $alpha in mathbbC$ with $|alpha -w_0|<epsilon$, $f(z)-alpha$ has exactly $m$ simple roots in $B(z_0,delta)$.




Looking for some Hints. I wish to use to use $$frac12pi iint_mathcalCfracf'(z)f(z)dz = textnumber of zeros of f text in Int(mathcalC)$$.







share|cite|improve this question





















  • $alpha = w_0$??
    – mathworker21
    Aug 3 at 9:26










  • you still have $alpha = w_0$
    – mathworker21
    Aug 3 at 9:28










  • just corrected it
    – Arindam
    Aug 3 at 9:28










  • was my answer below helpful?
    – mathworker21
    2 days ago










  • Yes, thanks. It was helpful.
    – Arindam
    yesterday












up vote
0
down vote

favorite









up vote
0
down vote

favorite












Let $f$ be analytic at $z_0 in mathbbC$ and $f(z_0)=w_0$. Suppose that $f(z)-w_0$ has a zero of finite order in $m geq 1$ at $z=z_0$.



We need to show that there exist $epsilon >0$ and $delta >0$ such that for all $alpha in mathbbC$ with $|alpha -w_0|<epsilon$, $f(z)-alpha$ has exactly $m$ simple roots in $B(z_0,delta)$.




Looking for some Hints. I wish to use to use $$frac12pi iint_mathcalCfracf'(z)f(z)dz = textnumber of zeros of f text in Int(mathcalC)$$.







share|cite|improve this question














Let $f$ be analytic at $z_0 in mathbbC$ and $f(z_0)=w_0$. Suppose that $f(z)-w_0$ has a zero of finite order in $m geq 1$ at $z=z_0$.



We need to show that there exist $epsilon >0$ and $delta >0$ such that for all $alpha in mathbbC$ with $|alpha -w_0|<epsilon$, $f(z)-alpha$ has exactly $m$ simple roots in $B(z_0,delta)$.




Looking for some Hints. I wish to use to use $$frac12pi iint_mathcalCfracf'(z)f(z)dz = textnumber of zeros of f text in Int(mathcalC)$$.









share|cite|improve this question












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edited Aug 3 at 14:11









Batominovski

22.6k22776




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asked Aug 3 at 9:25









Arindam

193113




193113











  • $alpha = w_0$??
    – mathworker21
    Aug 3 at 9:26










  • you still have $alpha = w_0$
    – mathworker21
    Aug 3 at 9:28










  • just corrected it
    – Arindam
    Aug 3 at 9:28










  • was my answer below helpful?
    – mathworker21
    2 days ago










  • Yes, thanks. It was helpful.
    – Arindam
    yesterday
















  • $alpha = w_0$??
    – mathworker21
    Aug 3 at 9:26










  • you still have $alpha = w_0$
    – mathworker21
    Aug 3 at 9:28










  • just corrected it
    – Arindam
    Aug 3 at 9:28










  • was my answer below helpful?
    – mathworker21
    2 days ago










  • Yes, thanks. It was helpful.
    – Arindam
    yesterday















$alpha = w_0$??
– mathworker21
Aug 3 at 9:26




$alpha = w_0$??
– mathworker21
Aug 3 at 9:26












you still have $alpha = w_0$
– mathworker21
Aug 3 at 9:28




you still have $alpha = w_0$
– mathworker21
Aug 3 at 9:28












just corrected it
– Arindam
Aug 3 at 9:28




just corrected it
– Arindam
Aug 3 at 9:28












was my answer below helpful?
– mathworker21
2 days ago




was my answer below helpful?
– mathworker21
2 days ago












Yes, thanks. It was helpful.
– Arindam
yesterday




Yes, thanks. It was helpful.
– Arindam
yesterday










2 Answers
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Well, you can use Rouché's theorem, which follows from the equation you wrote. $f(z)-alpha = (f(z)-w_0)-(w_0-alpha)$. You can take $delta > 0$ so that $z_0$ is the only root of $f$ in $B(z_0,2delta)$. Then take $epsilon = frac12sup_z in partial B(z_0,delta) |f(z)-w_0|$, so that $|w_0-alpha| le epsilon < |f(z)-w_0|$ for every $z in partial B(z_0,delta)$.






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    I assume that you have the condition $0<|alpha -w_0|<epsilon$; otherwise, as mathworker21 noted, $alpha=w_0$ will cause trouble. The problem also inherently assumes that $f$ is a nonconstant function.



    From the condition that $z=z_0$ is a root of $f(z)-w_0$ of order $m$, we can write
    $$f(z)=w_0+(z-z_0)^m,g(z),,$$
    where $g$ is a holomorphic function and $g(z_0)neq 0$. Since $g(z_0)neq0$, there exists a positive real number $delta_0$ such that, on the neighborhood $B_2delta_0(z_0)$ of $z_0$, $g$ does not vanish. Write $M>0$ for the minimum value of $|g|$ on the boundary $partial B_delta_0(z_0)$. Write $K$ for the maximum value of $|g'|$ on $partial B_delta_0(z_0)$. Set $$delta:=minleftdelta_0,fracMm,(K+1)right>0,.$$



    We first claim that, on $B_delta(z_0)$, $f'(z)$ has at most one root $z=z_0$. (This root has multiplicity $m-1$.) To see this, we note that
    $$beginalign
    f'(z)&=(z-z_0)^m,g'(z)+m,(z-z_0)^m-1,g(z)
    \&=(z-z_0)^m-1,big(m,(z-z_0),g'(z)+g(z)big),.
    endalign$$
    If $zin B_delta(z_0)setminusz_0$, then
    $$beginalignfracbig^m-1&=big|m,(z-z_0),g'(z)+g(z)big|geq big|g(z)big|-m,big|z-z_0big|,big|g'(z)big|
    \
    &geq M-m,delta, Kgeq M-left(fracMK+1right),K=fracMK+1>0,.
    endalign$$
    The claim is now established.



    Set $epsilon:=delta^m M>0$. For $alpha in B_delta(z_0)$, we see that
    $$f(z)-alpha=(z-z_0)^m,g(z)-(alpha-w_0),.$$
    When $alphain B_epsilon(w_0)$ and $zinpartial B_delta(z_0)$, we have
    $$big|(z-z_0)^m,g(z)big|geq delta^m M=epsilon> |alpha-w_0|,.$$
    Using Rouché's Theorem, the number of roots of $f(z)-alpha$ in $B_delta(z_0)$ is the same as the number of roots of $(z-z_0)^mg(z)$ in $B_delta(z_0)$, which is $m$.



    We now prove that, if $0<|alpha-w_0|<epsilon$, then $f(z)-alpha$ has no repeated roots inside $B_delta(z_0)$. First, by the choice of $delta$, the only roots of $f'(z)$ inside $B_delta(z_0)$ is $z_0$. Second, if $z=zetain B_delta(z_0)$ is a double root of $f(z)-alpha$, then both $f(zeta)-alpha$ and $left(dfractextdtextdz,big(f(z)-alphabig)right)Bigg|_z=zeta=f'(zeta)$ must be both $0$. However, $f'(zeta)=0$ implies $zeta=z_0$, but then $f(z_0)-alpha=w_0-alphaneq 0$. This is a contradiction, whence such $zeta in B_delta(z_0)$ does not exist.






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      2 Answers
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      Well, you can use Rouché's theorem, which follows from the equation you wrote. $f(z)-alpha = (f(z)-w_0)-(w_0-alpha)$. You can take $delta > 0$ so that $z_0$ is the only root of $f$ in $B(z_0,2delta)$. Then take $epsilon = frac12sup_z in partial B(z_0,delta) |f(z)-w_0|$, so that $|w_0-alpha| le epsilon < |f(z)-w_0|$ for every $z in partial B(z_0,delta)$.






      share|cite|improve this answer

























        up vote
        1
        down vote













        Well, you can use Rouché's theorem, which follows from the equation you wrote. $f(z)-alpha = (f(z)-w_0)-(w_0-alpha)$. You can take $delta > 0$ so that $z_0$ is the only root of $f$ in $B(z_0,2delta)$. Then take $epsilon = frac12sup_z in partial B(z_0,delta) |f(z)-w_0|$, so that $|w_0-alpha| le epsilon < |f(z)-w_0|$ for every $z in partial B(z_0,delta)$.






        share|cite|improve this answer























          up vote
          1
          down vote










          up vote
          1
          down vote









          Well, you can use Rouché's theorem, which follows from the equation you wrote. $f(z)-alpha = (f(z)-w_0)-(w_0-alpha)$. You can take $delta > 0$ so that $z_0$ is the only root of $f$ in $B(z_0,2delta)$. Then take $epsilon = frac12sup_z in partial B(z_0,delta) |f(z)-w_0|$, so that $|w_0-alpha| le epsilon < |f(z)-w_0|$ for every $z in partial B(z_0,delta)$.






          share|cite|improve this answer













          Well, you can use Rouché's theorem, which follows from the equation you wrote. $f(z)-alpha = (f(z)-w_0)-(w_0-alpha)$. You can take $delta > 0$ so that $z_0$ is the only root of $f$ in $B(z_0,2delta)$. Then take $epsilon = frac12sup_z in partial B(z_0,delta) |f(z)-w_0|$, so that $|w_0-alpha| le epsilon < |f(z)-w_0|$ for every $z in partial B(z_0,delta)$.







          share|cite|improve this answer













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          answered Aug 3 at 9:30









          mathworker21

          6,4231727




          6,4231727




















              up vote
              1
              down vote













              I assume that you have the condition $0<|alpha -w_0|<epsilon$; otherwise, as mathworker21 noted, $alpha=w_0$ will cause trouble. The problem also inherently assumes that $f$ is a nonconstant function.



              From the condition that $z=z_0$ is a root of $f(z)-w_0$ of order $m$, we can write
              $$f(z)=w_0+(z-z_0)^m,g(z),,$$
              where $g$ is a holomorphic function and $g(z_0)neq 0$. Since $g(z_0)neq0$, there exists a positive real number $delta_0$ such that, on the neighborhood $B_2delta_0(z_0)$ of $z_0$, $g$ does not vanish. Write $M>0$ for the minimum value of $|g|$ on the boundary $partial B_delta_0(z_0)$. Write $K$ for the maximum value of $|g'|$ on $partial B_delta_0(z_0)$. Set $$delta:=minleftdelta_0,fracMm,(K+1)right>0,.$$



              We first claim that, on $B_delta(z_0)$, $f'(z)$ has at most one root $z=z_0$. (This root has multiplicity $m-1$.) To see this, we note that
              $$beginalign
              f'(z)&=(z-z_0)^m,g'(z)+m,(z-z_0)^m-1,g(z)
              \&=(z-z_0)^m-1,big(m,(z-z_0),g'(z)+g(z)big),.
              endalign$$
              If $zin B_delta(z_0)setminusz_0$, then
              $$beginalignfracbig^m-1&=big|m,(z-z_0),g'(z)+g(z)big|geq big|g(z)big|-m,big|z-z_0big|,big|g'(z)big|
              \
              &geq M-m,delta, Kgeq M-left(fracMK+1right),K=fracMK+1>0,.
              endalign$$
              The claim is now established.



              Set $epsilon:=delta^m M>0$. For $alpha in B_delta(z_0)$, we see that
              $$f(z)-alpha=(z-z_0)^m,g(z)-(alpha-w_0),.$$
              When $alphain B_epsilon(w_0)$ and $zinpartial B_delta(z_0)$, we have
              $$big|(z-z_0)^m,g(z)big|geq delta^m M=epsilon> |alpha-w_0|,.$$
              Using Rouché's Theorem, the number of roots of $f(z)-alpha$ in $B_delta(z_0)$ is the same as the number of roots of $(z-z_0)^mg(z)$ in $B_delta(z_0)$, which is $m$.



              We now prove that, if $0<|alpha-w_0|<epsilon$, then $f(z)-alpha$ has no repeated roots inside $B_delta(z_0)$. First, by the choice of $delta$, the only roots of $f'(z)$ inside $B_delta(z_0)$ is $z_0$. Second, if $z=zetain B_delta(z_0)$ is a double root of $f(z)-alpha$, then both $f(zeta)-alpha$ and $left(dfractextdtextdz,big(f(z)-alphabig)right)Bigg|_z=zeta=f'(zeta)$ must be both $0$. However, $f'(zeta)=0$ implies $zeta=z_0$, but then $f(z_0)-alpha=w_0-alphaneq 0$. This is a contradiction, whence such $zeta in B_delta(z_0)$ does not exist.






              share|cite|improve this answer



























                up vote
                1
                down vote













                I assume that you have the condition $0<|alpha -w_0|<epsilon$; otherwise, as mathworker21 noted, $alpha=w_0$ will cause trouble. The problem also inherently assumes that $f$ is a nonconstant function.



                From the condition that $z=z_0$ is a root of $f(z)-w_0$ of order $m$, we can write
                $$f(z)=w_0+(z-z_0)^m,g(z),,$$
                where $g$ is a holomorphic function and $g(z_0)neq 0$. Since $g(z_0)neq0$, there exists a positive real number $delta_0$ such that, on the neighborhood $B_2delta_0(z_0)$ of $z_0$, $g$ does not vanish. Write $M>0$ for the minimum value of $|g|$ on the boundary $partial B_delta_0(z_0)$. Write $K$ for the maximum value of $|g'|$ on $partial B_delta_0(z_0)$. Set $$delta:=minleftdelta_0,fracMm,(K+1)right>0,.$$



                We first claim that, on $B_delta(z_0)$, $f'(z)$ has at most one root $z=z_0$. (This root has multiplicity $m-1$.) To see this, we note that
                $$beginalign
                f'(z)&=(z-z_0)^m,g'(z)+m,(z-z_0)^m-1,g(z)
                \&=(z-z_0)^m-1,big(m,(z-z_0),g'(z)+g(z)big),.
                endalign$$
                If $zin B_delta(z_0)setminusz_0$, then
                $$beginalignfracbig^m-1&=big|m,(z-z_0),g'(z)+g(z)big|geq big|g(z)big|-m,big|z-z_0big|,big|g'(z)big|
                \
                &geq M-m,delta, Kgeq M-left(fracMK+1right),K=fracMK+1>0,.
                endalign$$
                The claim is now established.



                Set $epsilon:=delta^m M>0$. For $alpha in B_delta(z_0)$, we see that
                $$f(z)-alpha=(z-z_0)^m,g(z)-(alpha-w_0),.$$
                When $alphain B_epsilon(w_0)$ and $zinpartial B_delta(z_0)$, we have
                $$big|(z-z_0)^m,g(z)big|geq delta^m M=epsilon> |alpha-w_0|,.$$
                Using Rouché's Theorem, the number of roots of $f(z)-alpha$ in $B_delta(z_0)$ is the same as the number of roots of $(z-z_0)^mg(z)$ in $B_delta(z_0)$, which is $m$.



                We now prove that, if $0<|alpha-w_0|<epsilon$, then $f(z)-alpha$ has no repeated roots inside $B_delta(z_0)$. First, by the choice of $delta$, the only roots of $f'(z)$ inside $B_delta(z_0)$ is $z_0$. Second, if $z=zetain B_delta(z_0)$ is a double root of $f(z)-alpha$, then both $f(zeta)-alpha$ and $left(dfractextdtextdz,big(f(z)-alphabig)right)Bigg|_z=zeta=f'(zeta)$ must be both $0$. However, $f'(zeta)=0$ implies $zeta=z_0$, but then $f(z_0)-alpha=w_0-alphaneq 0$. This is a contradiction, whence such $zeta in B_delta(z_0)$ does not exist.






                share|cite|improve this answer

























                  up vote
                  1
                  down vote










                  up vote
                  1
                  down vote









                  I assume that you have the condition $0<|alpha -w_0|<epsilon$; otherwise, as mathworker21 noted, $alpha=w_0$ will cause trouble. The problem also inherently assumes that $f$ is a nonconstant function.



                  From the condition that $z=z_0$ is a root of $f(z)-w_0$ of order $m$, we can write
                  $$f(z)=w_0+(z-z_0)^m,g(z),,$$
                  where $g$ is a holomorphic function and $g(z_0)neq 0$. Since $g(z_0)neq0$, there exists a positive real number $delta_0$ such that, on the neighborhood $B_2delta_0(z_0)$ of $z_0$, $g$ does not vanish. Write $M>0$ for the minimum value of $|g|$ on the boundary $partial B_delta_0(z_0)$. Write $K$ for the maximum value of $|g'|$ on $partial B_delta_0(z_0)$. Set $$delta:=minleftdelta_0,fracMm,(K+1)right>0,.$$



                  We first claim that, on $B_delta(z_0)$, $f'(z)$ has at most one root $z=z_0$. (This root has multiplicity $m-1$.) To see this, we note that
                  $$beginalign
                  f'(z)&=(z-z_0)^m,g'(z)+m,(z-z_0)^m-1,g(z)
                  \&=(z-z_0)^m-1,big(m,(z-z_0),g'(z)+g(z)big),.
                  endalign$$
                  If $zin B_delta(z_0)setminusz_0$, then
                  $$beginalignfracbig^m-1&=big|m,(z-z_0),g'(z)+g(z)big|geq big|g(z)big|-m,big|z-z_0big|,big|g'(z)big|
                  \
                  &geq M-m,delta, Kgeq M-left(fracMK+1right),K=fracMK+1>0,.
                  endalign$$
                  The claim is now established.



                  Set $epsilon:=delta^m M>0$. For $alpha in B_delta(z_0)$, we see that
                  $$f(z)-alpha=(z-z_0)^m,g(z)-(alpha-w_0),.$$
                  When $alphain B_epsilon(w_0)$ and $zinpartial B_delta(z_0)$, we have
                  $$big|(z-z_0)^m,g(z)big|geq delta^m M=epsilon> |alpha-w_0|,.$$
                  Using Rouché's Theorem, the number of roots of $f(z)-alpha$ in $B_delta(z_0)$ is the same as the number of roots of $(z-z_0)^mg(z)$ in $B_delta(z_0)$, which is $m$.



                  We now prove that, if $0<|alpha-w_0|<epsilon$, then $f(z)-alpha$ has no repeated roots inside $B_delta(z_0)$. First, by the choice of $delta$, the only roots of $f'(z)$ inside $B_delta(z_0)$ is $z_0$. Second, if $z=zetain B_delta(z_0)$ is a double root of $f(z)-alpha$, then both $f(zeta)-alpha$ and $left(dfractextdtextdz,big(f(z)-alphabig)right)Bigg|_z=zeta=f'(zeta)$ must be both $0$. However, $f'(zeta)=0$ implies $zeta=z_0$, but then $f(z_0)-alpha=w_0-alphaneq 0$. This is a contradiction, whence such $zeta in B_delta(z_0)$ does not exist.






                  share|cite|improve this answer















                  I assume that you have the condition $0<|alpha -w_0|<epsilon$; otherwise, as mathworker21 noted, $alpha=w_0$ will cause trouble. The problem also inherently assumes that $f$ is a nonconstant function.



                  From the condition that $z=z_0$ is a root of $f(z)-w_0$ of order $m$, we can write
                  $$f(z)=w_0+(z-z_0)^m,g(z),,$$
                  where $g$ is a holomorphic function and $g(z_0)neq 0$. Since $g(z_0)neq0$, there exists a positive real number $delta_0$ such that, on the neighborhood $B_2delta_0(z_0)$ of $z_0$, $g$ does not vanish. Write $M>0$ for the minimum value of $|g|$ on the boundary $partial B_delta_0(z_0)$. Write $K$ for the maximum value of $|g'|$ on $partial B_delta_0(z_0)$. Set $$delta:=minleftdelta_0,fracMm,(K+1)right>0,.$$



                  We first claim that, on $B_delta(z_0)$, $f'(z)$ has at most one root $z=z_0$. (This root has multiplicity $m-1$.) To see this, we note that
                  $$beginalign
                  f'(z)&=(z-z_0)^m,g'(z)+m,(z-z_0)^m-1,g(z)
                  \&=(z-z_0)^m-1,big(m,(z-z_0),g'(z)+g(z)big),.
                  endalign$$
                  If $zin B_delta(z_0)setminusz_0$, then
                  $$beginalignfracbig^m-1&=big|m,(z-z_0),g'(z)+g(z)big|geq big|g(z)big|-m,big|z-z_0big|,big|g'(z)big|
                  \
                  &geq M-m,delta, Kgeq M-left(fracMK+1right),K=fracMK+1>0,.
                  endalign$$
                  The claim is now established.



                  Set $epsilon:=delta^m M>0$. For $alpha in B_delta(z_0)$, we see that
                  $$f(z)-alpha=(z-z_0)^m,g(z)-(alpha-w_0),.$$
                  When $alphain B_epsilon(w_0)$ and $zinpartial B_delta(z_0)$, we have
                  $$big|(z-z_0)^m,g(z)big|geq delta^m M=epsilon> |alpha-w_0|,.$$
                  Using Rouché's Theorem, the number of roots of $f(z)-alpha$ in $B_delta(z_0)$ is the same as the number of roots of $(z-z_0)^mg(z)$ in $B_delta(z_0)$, which is $m$.



                  We now prove that, if $0<|alpha-w_0|<epsilon$, then $f(z)-alpha$ has no repeated roots inside $B_delta(z_0)$. First, by the choice of $delta$, the only roots of $f'(z)$ inside $B_delta(z_0)$ is $z_0$. Second, if $z=zetain B_delta(z_0)$ is a double root of $f(z)-alpha$, then both $f(zeta)-alpha$ and $left(dfractextdtextdz,big(f(z)-alphabig)right)Bigg|_z=zeta=f'(zeta)$ must be both $0$. However, $f'(zeta)=0$ implies $zeta=z_0$, but then $f(z_0)-alpha=w_0-alphaneq 0$. This is a contradiction, whence such $zeta in B_delta(z_0)$ does not exist.







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                  edited yesterday


























                  answered Aug 3 at 14:06









                  Batominovski

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