Added appropriate form of Taylor's formula.

This commit is contained in:
Bokuan Li
2026-05-18 16:24:43 -04:00
parent 3a0e5cc351
commit 85d1d78bda
2 changed files with 48 additions and 9 deletions

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@@ -221,7 +221,7 @@
\end{proof}
\begin{definition}[Complex Analytic]
\begin{definition}[Holomorphic]
\label{definition:complex-analytic}
Let $E$ be a complete separated locally convex space over $\complex$, $U \subset \complex$ be open, and $f \in C(U; E)$, then the following are equivalent:
\begin{enumerate}
@@ -234,16 +234,16 @@
\[
f(z_0) = \frac{1}{2\pi i} \int_\gamma \frac{f(z)}{z - z_0}dz
\]
\item (\textbf{Analyticity}) For each $z_0 \in U$ and $r > 0$ such that $\ol{B(z_0, r)} \subset U$, there exists $\seq{a_n} \subset E$ such that $f$ may be expressed as a power series
\item (\textbf{Analyticity}) For each $z_0 \in U$, there exists $r > 0$ and $\seq{a_n} \subset E$ such that for each $z \in B(z_0, r)$,
\[
f(z) = \sum_{n = 0}^\infty a_n(z - z_0)^n
\]
with radius of convergence at least $r$.
where the radius of convergence of the series is at least $r$.
\item (\textbf{Weak Holomorphy}) For each $\phi \in E^*$, $\phi \circ f$ satisfies the above.
\end{enumerate}
If the above holds, then $f$ is \textbf{complex analytic}.
If the above holds, then $f$ is \textbf{holomorphic/complex analytic} on $U$.
\end{definition}
\begin{proof}
(1) $\Leftrightarrow$ (2): \autoref{lemma:complex-analytic}.
@@ -267,7 +267,7 @@
Thus $[D^kf(z_0)/k!]_E \le C/r^k$ for all $k \in \natz$, and the radius of convergence of $g$ is at least $r$.
Let $z \in B(z_0, r/2)$, $s = |z - z_0|$, and $n \in \natp$, then by \hyperref[Taylor's Formula]{theorem:taylor-lagrange} and \hyperref[Cauchy's Estimate]{corollary:cauchy-estimate},
Let $z \in B(z_0, r/2)$, $s = |z - z_0|$, and $n \in \natp$, then by \hyperref[Taylor's Formula]{theorem:taylor-integral} and \hyperref[Cauchy's Estimate]{corollary:cauchy-estimate},
\begin{align*}
\braks{f(z) - \sum_{k = 0}^n \frac{1}{k!} D^kf(z_0)(z - z_0)^n}_E &\le s^{n+1} \cdot \sup_{z' \in \ol{B(z_0, s)}} [D^{n+1}f(z')]_E \\
&\le \frac{Cs^{n+1}}{(r-s)^{n+1}}