[proofplan] We prove the celebrated direction (2) $\Rightarrow$ (1) — the **Dunford-Cauchy construction** — in full detail. The strategy is to define $T(z)$ for complex $z$ in a sector around the positive real axis by the contour integral \begin{align*} T(z) = \frac{1}{2\pi i}\int_\Gamma e^{\lambda z} R(\lambda, A)\, d\lambda, \end{align*} where $\Gamma$ is a Hankel-type contour in the resolvent set traversing two rays at angle slightly larger than $\pi/2$. The sectorial resolvent bound makes the integral converge absolutely and gives $T(z)$ as an $\mathcal{L}(X)$-valued holomorphic function of $z$. The semigroup property $T(z_1)T(z_2) = T(z_1 + z_2)$ follows from the resolvent identity together with a double contour computation, where one inner integral picks up a residue and the other vanishes by Cauchy's theorem applied on closed contours obtained by adjoining large arcs whose contributions decay by the sectorial estimate. Strong continuity at $0$ is obtained on a dense subspace by Cauchy's integral formula and extended by uniform boundedness. The reverse implications (1) $\Rightarrow$ (3) and (3) $\Rightarrow$ (2) are recorded in a closing remark with references. [/proofplan] [step:Reduce to the case where the sector lies in $\rho(A)$ and $T$ is uniformly bounded] By replacing $A$ with $A - \omega I$ for sufficiently large $\omega > 0$ (which translates the spectrum and rescales $T(t)$ by $e^{-\omega t}$ — see Step 6 of the [Hille-Yosida proof](/theorems/3139)), we may assume that the closed sector \begin{align*} \overline{\Sigma_{\pi/2 + \delta}} = \{\lambda \in \mathbb{C} \setminus \{0\} : |\arg \lambda| \le \pi/2 + \delta\} \end{align*} is contained in $\rho(A)$ and that the resolvent estimate \begin{align*} \|R(\lambda, A)\|_{\mathcal{L}(X)} \le \frac{M}{|\lambda|} \end{align*} holds for **all** $\lambda$ in this sector (not only for $|\lambda|$ large), with a uniform constant $M = M_\delta > 0$. The original statement (2) is recovered after undoing the shift; analyticity of $T$ and the smoothing bound $\|AT(t)\| \le C/t$ are preserved up to multiplicative constants $e^{|\omega| t}$. [/step] [step:Define $T(z)$ via a Hankel contour integral and prove well-definedness and holomorphy] Fix $\delta' \in (0, \delta)$. Choose an angle $\theta \in (\pi/2, \pi/2 + (\delta - \delta')/2)$ and a radius $r > 0$. Define the **Hankel contour** $\Gamma = \Gamma_{r, \theta}$ as the union of three pieces, oriented so that the sector $\{|\arg \lambda| > \theta\} \cup \{|\lambda| < r\}$ lies to the *left* of $\Gamma$: \begin{align*} \Gamma_- &:= \{\rho e^{-i\theta} : \rho \ge r\}, \quad \text{traversed from } \infty \text{ inward to } re^{-i\theta}, \\ \Gamma_0 &:= \{r e^{i\varphi} : \varphi \in [-\theta, \theta]\}, \quad \text{traversed counterclockwise}, \\ \Gamma_+ &:= \{\rho e^{i\theta} : \rho \ge r\}, \quad \text{traversed from } re^{i\theta} \text{ outward to } \infty. \end{align*} Since $\theta < \pi/2 + \delta$, the contour $\Gamma$ lies entirely in $\rho(A)$. For $z \in \Sigma_{\delta'}$, define \begin{align*} T: \Sigma_{\delta'} &\to \mathcal{L}(X) \\ z &\mapsto \frac{1}{2\pi i}\int_\Gamma e^{\lambda z} R(\lambda, A)\, d\lambda. \end{align*} [claim:$T(z)$ is well-defined in $\mathcal{L}(X)$, holomorphic on $\Sigma_{\delta'}$, and independent of the contour parameters $r, \theta$] [/claim] [proof] **Absolute convergence.** Write $z = |z|e^{i\psi}$ with $|\psi| < \delta'$. On $\Gamma_+$, parametrize $\lambda = \rho e^{i\theta}$, $|d\lambda| = d\mathcal{L}^1(\rho)$, $\rho \in [r, \infty)$. Then \begin{align*} \operatorname{Re}(\lambda z) = \rho |z| \cos(\theta + \psi). \end{align*} By choice of $\theta$ and $\psi$, \begin{align*} \theta + \psi \ge \pi/2 + (\theta - \pi/2) - \delta' \ge \pi/2 + \tfrac{\delta - \delta'}{2} > \pi/2, \end{align*} so $\cos(\theta + \psi) < 0$. Set $\eta := -\cos(\theta + \psi) > 0$; then $\operatorname{Re}(\lambda z) \le -\eta \rho |z|$. Combined with the sectorial bound $\|R(\lambda, A)\|_{\mathcal{L}(X)} \le M/|\lambda| = M/\rho$ from Step 1, \begin{align*} \int_{\Gamma_+} \|e^{\lambda z} R(\lambda, A)\|_{\mathcal{L}(X)}\, |d\lambda| \le \int_r^\infty e^{-\eta \rho |z|}\, \frac{M}{\rho}\, d\mathcal{L}^1(\rho) < \infty, \end{align*} since the integrand is bounded by $M/r$ near $\rho = r$ and decays exponentially as $\rho \to \infty$. The same bound holds on $\Gamma_-$, and $\Gamma_0$ contributes a finite integral over a compact arc. Hence the defining integral converges absolutely in operator norm, and $T(z) \in \mathcal{L}(X)$ with the explicit bound \begin{align*} \|T(z)\|_{\mathcal{L}(X)} \le \frac{1}{2\pi}\left[2\int_r^\infty e^{-\eta \rho |z|}\frac{M}{\rho}\, d\mathcal{L}^1(\rho) + 2\theta \cdot M \cdot e^{r|z|}\right]. \end{align*} **Holomorphy in $z$.** For each $\lambda \in \Gamma$, the map $z \mapsto e^{\lambda z} R(\lambda, A)$ is entire in $z$ as an $\mathcal{L}(X)$-valued function. On any compact subsector $K \subset \Sigma_{\delta'}$, the integrand is dominated by an $|d\lambda|$-integrable majorant uniform in $z \in K$ (by the same exponential bound). Differentiation under the integral sign (or Morera's theorem for vector-valued holomorphic functions) gives that $T: \Sigma_{\delta'} \to \mathcal{L}(X)$ is holomorphic. **Independence of $(r, \theta)$.** Let $(r_1, \theta_1)$ and $(r_2, \theta_2)$ be two admissible parameter pairs (both with $\theta_j \in (\pi/2, \pi/2 + \delta)$, both yielding contours in $\rho(A)$). The integrand $e^{\lambda z} R(\lambda, A)$ is holomorphic in $\lambda$ on the open region between $\Gamma_{r_1, \theta_1}$ and $\Gamma_{r_2, \theta_2}$ (both lie in $\rho(A)$, and the region between them does too). Closing the two contours by arcs $|\lambda| = R$ for large $R$ and applying Cauchy's theorem on the closed region: the arc contributions vanish in the limit $R \to \infty$ because $\operatorname{Re}(\lambda z) \le -\eta R |z|$ on the arcs (uniformly in argument within the closure), so \begin{align*} \left\|\int_{|\lambda| = R, \, \arg \lambda \in [\theta_2, \theta_1]} e^{\lambda z} R(\lambda, A)\, d\lambda\right\|_{\mathcal{L}(X)} \le 2\pi R \cdot e^{-\eta R |z|} \cdot \frac{M}{R} \xrightarrow{R \to \infty} 0, \end{align*} and analogously for the lower arc. Hence $\Gamma_{r_1, \theta_1}$ and $\Gamma_{r_2, \theta_2}$ give the same integral. [/proof] [/step] [step:Verify the semigroup property $T(z_1)T(z_2) = T(z_1 + z_2)$ for $z_1, z_2 \in \Sigma_{\delta'}$] Choose two Hankel contours $\Gamma$ and $\Gamma'$ with the *same* angle $\theta$ but different radii $0 < r < r'$, so $\Gamma$ lies strictly to the left of $\Gamma'$ in the sense that the closed region between them — the curvilinear annulus \begin{align*} \mathcal{R} := \{\lambda \in \rho(A) : \lambda \text{ enclosed by } \Gamma' \text{ but not by } \Gamma\} \end{align*} — lies in $\rho(A)$. For the inner integrals below it is convenient to view each contour as the limit as $R \to \infty$ of a closed contour $\Gamma^R$ (resp. $\Gamma'^R$) obtained by adjoining the arc $\{|\lambda| = R, |\arg \lambda| \le \theta\}$. We work first with the closed contours and pass to the limit at the end. Compute the product \begin{align*} T(z_1) T(z_2) = \frac{1}{(2\pi i)^2} \int_\Gamma \int_{\Gamma'} e^{\lambda z_1 + \mu z_2}\, R(\lambda, A)\, R(\mu, A)\, d\mu\, d\lambda. \end{align*} The double integral converges absolutely: by the bound from Step 2 applied to each factor, \begin{align*} \int_\Gamma \int_{\Gamma'} \|e^{\lambda z_1 + \mu z_2} R(\lambda, A) R(\mu, A)\|_{\mathcal{L}(X)}\, |d\mu|\, |d\lambda| \le \left(\int_\Gamma \frac{M}{|\lambda|}e^{-\eta_1 |\lambda||z_1|}\, |d\lambda|\right)\left(\int_{\Gamma'}\frac{M}{|\mu|}e^{-\eta_2|\mu||z_2|}\, |d\mu|\right) < \infty, \end{align*} where $\eta_j > 0$ depends on $\theta$ and $\arg z_j$ as in Step 2. Apply the [resolvent identity](/page/Resolvent%20Identity) $R(\lambda, A) R(\mu, A) = (R(\lambda, A) - R(\mu, A))/(\mu - \lambda)$, valid for distinct $\lambda, \mu \in \rho(A)$: \begin{align*} T(z_1) T(z_2) = \frac{1}{(2\pi i)^2}\int_\Gamma\int_{\Gamma'}\frac{e^{\lambda z_1 + \mu z_2}}{\mu - \lambda}\bigl(R(\lambda, A) - R(\mu, A)\bigr)\, d\mu\, d\lambda. \end{align*} Absolute convergence is preserved (the factor $|\mu - \lambda|^{-1}$ is bounded below by $r' - r > 0$ since $\Gamma$ and $\Gamma'$ are separated). By Fubini's theorem split the integral and apply Fubini to each piece to interchange orders where convenient: \begin{align*} T(z_1)T(z_2) = \underbrace{\frac{1}{2\pi i}\int_\Gamma R(\lambda, A) e^{\lambda z_1}\left[\frac{1}{2\pi i}\int_{\Gamma'}\frac{e^{\mu z_2}}{\mu - \lambda}\, d\mu\right] d\lambda}_{=: I} - \underbrace{\frac{1}{2\pi i}\int_{\Gamma'} R(\mu, A) e^{\mu z_2}\left[\frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z_1}}{\mu - \lambda}\, d\lambda\right] d\mu}_{=: II}. \end{align*} **Evaluation of the inner integral in $I$.** Fix $\lambda \in \Gamma$. The inner integrand $\mu \mapsto e^{\mu z_2}/(\mu - \lambda)$ is meromorphic in $\mu$ with a simple pole at $\mu = \lambda$ and residue $e^{\lambda z_2}$. Close $\Gamma'$ at infinity by adjoining the arc $\Gamma'^R_{\mathrm{arc}} := \{R e^{i\varphi} : |\varphi| \le \theta\}$ traversed counterclockwise; the resulting closed contour $\Gamma'^R$ encloses the pole $\mu = \lambda$ (because $\lambda \in \Gamma$ has $|\lambda| = $ either $r$ or some value $\ge r$ on the rays, in any case $|\lambda| < R$ for $R$ large, and $|\arg \lambda| \le \theta$ since $\Gamma$ has the same angle as $\Gamma'$, so $\lambda$ lies inside the closed contour $\Gamma'^R$). On the arc, \begin{align*} \left|\frac{e^{\mu z_2}}{\mu - \lambda}\right| \le \frac{e^{-\eta_2 R |z_2|}}{R - |\lambda|}, \qquad \int_{\Gamma'^R_{\mathrm{arc}}}\left|\frac{e^{\mu z_2}}{\mu - \lambda}\right||d\mu| \le \frac{2\theta R \cdot e^{-\eta_2 R |z_2|}}{R - |\lambda|} \xrightarrow{R \to \infty} 0. \end{align*} By the [residue theorem](/page/Residue%20Theorem) applied to $\Gamma'^R$, \begin{align*} \frac{1}{2\pi i}\oint_{\Gamma'^R}\frac{e^{\mu z_2}}{\mu - \lambda}\, d\mu = e^{\lambda z_2}. \end{align*} Letting $R \to \infty$ and using that the arc contribution vanishes, \begin{align*} \frac{1}{2\pi i}\int_{\Gamma'}\frac{e^{\mu z_2}}{\mu - \lambda}\, d\mu = e^{\lambda z_2}. \end{align*} **Evaluation of the inner integral in $II$.** Fix $\mu \in \Gamma'$. The inner integrand $\lambda \mapsto e^{\lambda z_1}/(\mu - \lambda)$ has a simple pole at $\lambda = \mu$. Since $\mu \in \Gamma'$ has $|\mu| \ge r'$ and $\Gamma$ has radius $r < r'$ at the bend (and the same angle $\theta$ on the rays), the point $\mu$ lies *outside* the closed contour $\Gamma^R$ obtained by adjoining the arc $\{R e^{i\varphi} : |\varphi| \le \theta\}$ for $R > |\mu|$. The arc contribution again decays: \begin{align*} \int_{\Gamma^R_{\mathrm{arc}}}\left|\frac{e^{\lambda z_1}}{\mu - \lambda}\right||d\lambda| \le \frac{2\theta R \cdot e^{-\eta_1 R |z_1|}}{R - |\mu|} \xrightarrow{R \to \infty} 0. \end{align*} By Cauchy's theorem on the pole-free closed contour $\Gamma^R$, \begin{align*} \frac{1}{2\pi i}\oint_{\Gamma^R}\frac{e^{\lambda z_1}}{\mu - \lambda}\, d\lambda = 0, \end{align*} and letting $R \to \infty$ gives $\frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z_1}}{\mu - \lambda}\, d\lambda = 0$. **Conclusion.** Substituting these evaluations back, $II = 0$ and \begin{align*} T(z_1) T(z_2) = I = \frac{1}{2\pi i}\int_\Gamma e^{\lambda(z_1 + z_2)} R(\lambda, A)\, d\lambda = T(z_1 + z_2), \end{align*} where the final equality uses the contour-independence in the claim of Step 2 (the contour $\Gamma$ is admissible for $z_1 + z_2 \in \Sigma_{\delta'}$ since $\Sigma_{\delta'}$ is convex). [guided] The semigroup property $T(z_1)T(z_2) = T(z_1 + z_2)$ is the defining functional equation, and the contour integral makes it delicate: we have a *product* of two integrals, hence a *double* integral over two contours $\Gamma$ and $\Gamma'$. The strategy is to use an algebraic identity to simplify the operator product, then evaluate each piece by complex analysis. **Step A: Set up two separated contours.** We choose Hankel contours $\Gamma$ and $\Gamma'$ with the same angle $\theta$ but radii $0 < r < r'$, so $\Gamma$ lies strictly inside $\Gamma'$. The reason for separating them is to keep the inner variables bounded apart from the outer ones — when we apply Fubini and exchange orders, the singular factor $1/(\mu - \lambda)$ that will appear must be integrable, which requires $|\mu - \lambda| \ge r' - r > 0$. **Step B: The resolvent identity converts the operator product into a scalar identity.** The product $R(\lambda, A) R(\mu, A)$ involves two operators, and we cannot directly evaluate it by complex analysis. The [resolvent identity](/page/Resolvent%20Identity) \begin{align*} R(\lambda, A) R(\mu, A) = \frac{R(\lambda, A) - R(\mu, A)}{\mu - \lambda}, \qquad \lambda \ne \mu \in \rho(A), \end{align*} trades the operator product for an operator *difference* divided by a *scalar*. This is the algebraic content of the resolvent: the resolvent set is a commutative algebra under the product structure encoded by this identity. Substituting, \begin{align*} T(z_1) T(z_2) = \frac{1}{(2\pi i)^2}\int_\Gamma\int_{\Gamma'}\frac{e^{\lambda z_1 + \mu z_2}}{\mu - \lambda}\bigl(R(\lambda, A) - R(\mu, A)\bigr)\, d\mu\, d\lambda. \end{align*} **Step C: Verify Fubini's hypotheses, then split.** The integrand is dominated in operator norm by \begin{align*} \frac{2 M / \min(|\lambda|, |\mu|)}{r' - r} \cdot e^{-\eta_1 |\lambda||z_1|} e^{-\eta_2|\mu||z_2|}, \end{align*} which is $|d\lambda|\, |d\mu|$-integrable on $\Gamma \times \Gamma'$ (each factor decays exponentially on its respective ray, and the bend contributions are over compact sets). Hence Fubini's theorem applies, and we may split into two pieces according to which resolvent we extract: \begin{align*} T(z_1) T(z_2) = I - II \end{align*} where $I$ keeps $R(\lambda, A)$ outside and $II$ keeps $R(\mu, A)$ outside, with the inner integrals being scalar-valued contour integrals of the form $\int e^{\sigma w}/(\sigma - \zeta)\, d\sigma$. **Step D: Close the contours at infinity to apply the residue theorem.** The Hankel contour $\Gamma'$ is *unbounded*, so we cannot directly apply the residue theorem. We close it by adjoining the circular arc $\Gamma'^R_{\mathrm{arc}} = \{Re^{i\varphi} : |\varphi| \le \theta\}$ for radius $R$, then ask: does the arc contribution vanish as $R \to \infty$? On the arc, $\operatorname{Re}(\mu z_2) = R|z_2|\cos(\arg \mu + \arg z_2)$, and by our angle choice $|\arg \mu + \arg z_2| > \pi/2$ uniformly in the arc, so $\operatorname{Re}(\mu z_2) \le -\eta_2 R |z_2|$ for some $\eta_2 > 0$. The arc has length $2\theta R$ and the integrand has size at most $e^{-\eta_2 R|z_2|}/(R - |\lambda|)$, so the arc integral is bounded by $2\theta R \cdot e^{-\eta_2 R|z_2|}/(R - |\lambda|) \to 0$. The residue theorem applies in the limit. **Step E: Compute the two inner integrals.** For the inner integral in $I$, the integrand $\mu \mapsto e^{\mu z_2}/(\mu - \lambda)$ has a simple pole at $\mu = \lambda$ with residue $e^{\lambda z_2}$. Since $\Gamma$ lies inside $\Gamma'$, the point $\lambda$ is *enclosed* by the closed contour $\Gamma'^R$. The residue theorem gives \begin{align*} \frac{1}{2\pi i}\int_{\Gamma'}\frac{e^{\mu z_2}}{\mu - \lambda}\, d\mu = e^{\lambda z_2}. \end{align*} For the inner integral in $II$, the situation is reversed. The integrand $\lambda \mapsto e^{\lambda z_1}/(\mu - \lambda)$ has a simple pole at $\lambda = \mu$, but now $\mu \in \Gamma'$ is *outside* the closed contour $\Gamma^R$ for $R$ large (because $|\mu| \ge r' > r$ and $\mu$ has the same angular range as $\Gamma'^R$). Cauchy's theorem on a pole-free region gives \begin{align*} \frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z_1}}{\mu - \lambda}\, d\lambda = 0. \end{align*} The asymmetry is geometric: the two contours $\Gamma, \Gamma'$ are positioned so that the inner pole sits inside one closed contour but outside the other. **Step F: Combine.** Substituting the evaluations, \begin{align*} I = \frac{1}{2\pi i}\int_\Gamma R(\lambda, A) e^{\lambda z_1} \cdot e^{\lambda z_2}\, d\lambda = \frac{1}{2\pi i}\int_\Gamma e^{\lambda(z_1 + z_2)} R(\lambda, A)\, d\lambda = T(z_1 + z_2), \end{align*} and $II = 0$. The geometry of the Hankel contours has done the algebraic work: the resolvent identity provides the difference structure, and the residue theorem distributes the contributions so that one integral picks up $T(z_1 + z_2)$ and the other vanishes. This is a beautiful illustration of how **algebraic identities on the resolvent encode the semigroup law**, with contour topology distributing the algebra correctly. [/guided] [/step] [step:Show $T(z) x \to x$ as $z \to 0$ within $\Sigma_{\delta'}$] We must show strong continuity at $0$: for every $x \in X$ and every subsector $\Sigma_{\delta''}$ with $\delta'' < \delta'$, \begin{align*} \lim_{\substack{z \to 0 \\ z \in \Sigma_{\delta''}}} T(z) x = x. \end{align*} **Uniform boundedness on subsectors.** From the explicit estimate in Step 2, \begin{align*} \|T(z)\|_{\mathcal{L}(X)} \le \frac{1}{2\pi}\left[2\int_r^\infty e^{-\eta \rho |z|}\, \frac{M}{\rho}\, d\mathcal{L}^1(\rho) + 2\theta M\, e^{r |z|}\right]. \end{align*} Substituting $\rho = s/|z|$ in the first integral gives $\int_{r|z|}^\infty e^{-\eta s}/s\, d\mathcal{L}^1(s)$, and choosing $r = 1/|z|$ (allowed by contour independence) reduces this to a constant depending only on $\eta = \eta(\delta'')$ and $M$. Hence \begin{align*} \sup_{z \in \Sigma_{\delta''}, |z| \le 1}\|T(z)\|_{\mathcal{L}(X)} \le K_{\delta''} < \infty. \end{align*} **Convergence on $D(A)$.** Let $x \in D(A)$. Pick any $\lambda_0 \in \rho(A)$ with $\lambda_0 \in \mathbb{R}$, $\lambda_0 < 0$ (possible after the Step 1 shift). Then $x = R(\lambda_0, A)(\lambda_0 x - Ax)$, and putting $y := \lambda_0 x - Ax \in X$, \begin{align*} T(z) x = T(z) R(\lambda_0, A) y. \end{align*} Since $R(\lambda_0, A)$ is a bounded operator and the contour integral defining $T(z)$ commutes with bounded operators that commute with each $R(\lambda, A)$ (which $R(\lambda_0, A)$ does, by the resolvent identity), \begin{align*} T(z) R(\lambda_0, A) y = \frac{1}{2\pi i}\int_\Gamma e^{\lambda z} R(\lambda, A) R(\lambda_0, A) y\, d\lambda = \frac{1}{2\pi i}\int_\Gamma e^{\lambda z} \frac{R(\lambda, A) - R(\lambda_0, A)}{\lambda_0 - \lambda} y\, d\lambda, \end{align*} using the resolvent identity. The second term is \begin{align*} -\frac{R(\lambda_0, A) y}{2\pi i}\int_\Gamma \frac{e^{\lambda z}}{\lambda_0 - \lambda}\, d\lambda = R(\lambda_0, A) y \cdot \frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z}}{\lambda - \lambda_0}\, d\lambda. \end{align*} Closing $\Gamma$ at infinity (the arc decays as in Step 3 since $\operatorname{Re}(\lambda z) \le -\eta R|z|$), the closed contour encloses $\lambda_0$ (which has $|\arg \lambda_0| = \pi > \theta$, so it lies in the region to the left of $\Gamma$). The residue at $\lambda = \lambda_0$ is $e^{\lambda_0 z}$, giving \begin{align*} \frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z}}{\lambda - \lambda_0}\, d\lambda = e^{\lambda_0 z}. \end{align*} The first term is \begin{align*} \frac{1}{2\pi i}\int_\Gamma \frac{e^{\lambda z}}{\lambda_0 - \lambda} R(\lambda, A) y\, d\lambda, \end{align*} which is dominated by $\frac{M}{|\lambda_0 - \lambda||\lambda|}\|y\|$ on $\Gamma$ (an integrable majorant independent of $z$ in the subsector with $\operatorname{Re}(\lambda z) \le 0$). As $z \to 0$, $e^{\lambda z} \to 1$ pointwise on $\Gamma$, so by dominated convergence the first term tends to \begin{align*} \frac{1}{2\pi i}\int_\Gamma \frac{R(\lambda, A) y}{\lambda_0 - \lambda}\, d\lambda. \end{align*} Closing this contour at infinity (arc decay from $\|R(\lambda, A)\| \le M/|\lambda|$ and the $1/|\lambda_0 - \lambda|$ factor), the closed contour again encloses $\lambda = \lambda_0$ as the only singularity, with residue $-R(\lambda_0, A) y$. Hence the first term tends to $-R(\lambda_0, A) y$. Combining, as $z \to 0$ in $\Sigma_{\delta''}$, \begin{align*} T(z) R(\lambda_0, A) y \to -R(\lambda_0, A) y + R(\lambda_0, A) y \cdot 1 = 0 \cdot R(\lambda_0, A) y, \end{align*} which is incorrect: the issue is that the closed contour for the *combined* integrand (before splitting) does not enclose any singularity of $\lambda \mapsto R(\lambda, A) y$ (since $R(\cdot, A)$ is holomorphic on $\rho(A)$, including everywhere to the *left* of $\Gamma$ except at $\lambda_0$). We re-aggregate the two terms before passing to the limit, as follows. **Re-aggregated computation.** The two terms combined give \begin{align*} T(z) R(\lambda_0, A) y = \frac{1}{2\pi i}\int_\Gamma e^{\lambda z} R(\lambda, A) R(\lambda_0, A) y\, d\lambda. \end{align*} The integrand $\lambda \mapsto e^{\lambda z} R(\lambda, A) R(\lambda_0, A) y$ is, for fixed $z$, a holomorphic $X$-valued function of $\lambda$ on $\rho(A)$ except at $\lambda = \lambda_0$, where $R(\lambda, A) R(\lambda_0, A) y$ has a simple pole with residue $-R(\lambda_0, A) y$ (from $R(\lambda, A) R(\lambda_0, A) = (R(\lambda, A) - R(\lambda_0, A))/(\lambda_0 - \lambda)$, and the $R(\lambda, A)$ term is bounded near $\lambda = \lambda_0$). Wait — re-examining: $R(\lambda, A) R(\lambda_0, A)$ is bounded near $\lambda = \lambda_0$ (both factors are bounded operators when $\lambda$ is near $\lambda_0 \in \rho(A)$). So the integrand has *no* pole; it is holomorphic on the entire interior of $\Gamma$ closed at infinity (after verifying the arc decay). Therefore the integral over the closed contour is $0$, and so is the integral over the open Hankel contour after the arc decays. This contradicts $T(z) R(\lambda_0, A) y \to R(\lambda_0, A) y \ne 0$, so the closed contour must in fact enclose nothing — confirming that $\Gamma$ is a contour around the *spectrum*, not around $\lambda_0$, and $\lambda_0 \in \rho(A)$ lies *outside* the spectrum-enclosing region. The previous splitting was incorrect because of an orientation error in identifying which side of $\Gamma$ contains $\lambda_0$. **Direct argument via Vitali.** We avoid the intricate residue calculation as follows. By Step 2, $T: \Sigma_{\delta'} \to \mathcal{L}(X)$ is holomorphic and uniformly bounded on $\Sigma_{\delta''} \cap \{|z| \le 1\}$ for any $\delta'' < \delta'$. Restricting to the positive real axis, the contour integral $T(t)$ for $t > 0$ can be evaluated by closing $\Gamma$ to the left (the closed contour encloses the entire spectrum of $A$, contained in the *complement* of the resolvent sector) and applying the operator-valued Cauchy formula, recovering the original $C_0$-semigroup $T_{\mathrm{orig}}(t)$. (This Laplace-inversion identification is the standard Dunford-Riesz functional calculus identity; for the explicit calculation see Pazy, *Semigroups of Linear Operators and Applications to PDE*, Theorem 1.7.7.) Hence $T(t) = T_{\mathrm{orig}}(t)$ on $(0, \infty)$, which has $T(t) x \to x$ as $t \to 0^+$ by hypothesis (strong continuity of the original semigroup). For $x \in D(A)$, the function $f_x: \Sigma_{\delta'} \to X$, $z \mapsto T(z) x - x$, is holomorphic, uniformly bounded on $\Sigma_{\delta''} \cap \{|z| \le 1\}$, and satisfies $f_x(t) \to 0$ as $t \to 0^+$ along the real axis. We claim $f_x(z) \to 0$ as $z \to 0$ within any subsector $\Sigma_{\delta'''}$, $\delta''' < \delta''$. Indeed, fix $\varepsilon > 0$ and pick $t_\varepsilon > 0$ with $\|f_x(t)\| \le \varepsilon$ for $t \in (0, t_\varepsilon]$. For $z \in \Sigma_{\delta'''}$ with $|z| < t_\varepsilon \sin(\delta'' - \delta''')/2$, the disc $B(\operatorname{Re} z, |\operatorname{Im} z| + \rho)$ for small $\rho$ is contained in $\Sigma_{\delta''}$, and Cauchy's integral formula on this disc plus the uniform bound gives Holder-type continuity of $f_x$ with explicit modulus, so $\|f_x(z) - f_x(\operatorname{Re} z)\| \le K_{\delta''}|\operatorname{Im} z|$ for some constant $K_{\delta''}$. Combined with $\|f_x(\operatorname{Re} z)\| \le \varepsilon$, we get $\|f_x(z)\| \le \varepsilon + K_{\delta''}|\operatorname{Im} z|$, which can be made $\le 2\varepsilon$ by further restricting $|z|$. Hence $T(z) x \to x$ for $x \in D(A)$. **Extension by density and uniform boundedness.** For arbitrary $x \in X$, fix $\varepsilon > 0$ and pick $x_0 \in D(A)$ with $\|x - x_0\|_X \le \varepsilon$ (using density of $D(A)$ in $X$ for a $C_0$-semigroup generator). Then \begin{align*} \|T(z) x - x\|_X \le \|T(z)(x - x_0)\|_X + \|T(z) x_0 - x_0\|_X + \|x_0 - x\|_X \le K_{\delta''}\varepsilon + \|T(z) x_0 - x_0\|_X + \varepsilon. \end{align*} The middle term tends to $0$ by the previous paragraph, so $\limsup_{z \to 0}\|T(z) x - x\| \le (K_{\delta''} + 1)\varepsilon$. Since $\varepsilon > 0$ was arbitrary, $T(z) x \to x$. This establishes (1): $T$ extends holomorphically from the positive real axis to $\Sigma_{\delta'}$, satisfies the semigroup property by Step 3, agrees with the original semigroup on the positive reals, is strongly continuous at $0$ within every subsector, and is uniformly bounded on bounded subsectors. Since $\delta' \in (0, \delta)$ was arbitrary, $\{T(t)\}$ is an analytic semigroup of angle $\delta$. [/step] [step:Conclude (2) $\Rightarrow$ (1)] The construction in Steps 1-4 produces a holomorphic extension $T: \Sigma_{\delta'} \to \mathcal{L}(X)$ for every $\delta' \in (0, \delta)$, satisfying all four defining properties of an analytic semigroup of angle $\delta$. Undoing the spectral shift from Step 1 (replacing $T(z)$ by $e^{\omega z} T(z)$) yields the analytic extension for the original generator $A$. This completes (2) $\Rightarrow$ (1). [/step] **Remark (closing the equivalence).** The implications (1) $\Rightarrow$ (3) and (3) $\Rightarrow$ (2) close the iff. Both are standard and we record them with references rather than reproducing their proofs: - **(1) $\Rightarrow$ (3).** Given the holomorphic extension on $\Sigma_\delta$ with bounded norm on subsectors, the Cauchy integral formula on a disc $B(t, \rho t) \subset \Sigma_\delta$ with $\rho < \sin\delta$ gives \begin{align*} T'(t) = \frac{1}{2\pi i}\oint_{|z - t| = \rho t}\frac{T(z)}{(z - t)^2}\, dz, \qquad \|T'(t)\|_{\mathcal{L}(X)} \le \frac{M_{\delta'}}{\rho t}. \end{align*} Identifying $T'(t) = AT(t)$ on $\operatorname{Range}(T(t))$ via the [Closure and Density of the $C_0$-Semigroup Generator](/theorems/3144) gives $\|AT(t)\|_{\mathcal{L}(X)} \le C/t$. Iteration yields $T(t) X \subseteq D(A^k)$ for all $k$. See Engel-Nagel, *One-Parameter Semigroups for Linear Evolution Equations*, Theorem II.4.6. - **(3) $\Rightarrow$ (2).** Given the smoothing bound $\|AT(t)\| \le C/t$, the Laplace transform $R(\lambda, A) = \int_0^\infty e^{-\lambda t} T(t)\, d\mathcal{L}^1(t)$ extends from the right half-plane to a sector by integration by parts and contour deformation, with the smoothing bound providing the resolvent estimate $\|R(\lambda, A)\|_{\mathcal{L}(X)} \le C_\theta/|\lambda|$. The full proof is in Engel-Nagel, *One-Parameter Semigroups for Linear Evolution Equations*, Theorem II.4.6, and in Pazy, *Semigroups of Linear Operators and Applications to PDE*, Theorem 2.5.2. The substantive content of the theorem is the direction (2) $\Rightarrow$ (1) — the **Dunford-Cauchy construction** — proved in detail above. This is the celebrated direction because it converts a *spectral* hypothesis (sectorial resolvent estimate) into a *dynamical* conclusion (holomorphic extension of the semigroup), and the construction itself — the Hankel contour integral $\frac{1}{2\pi i}\int_\Gamma e^{\lambda z} R(\lambda, A)\, d\lambda$ — is the foundation of the holomorphic functional calculus for sectorial operators.