# Fundamental Theorem of Calculus for Contour Integrals

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## Theorem

Let $F, f: D \to \C$ be complex functions, where $D$ is a connected domain.

Let $C$ be a contour that is a concatenation of the directed smooth curves $C_1, \ldots, C_n$.

Let $C_k$ be parameterized by the smooth path $\gamma_k: \closedint {a_k} {b_k} \to D$ for all $k \in \set {1, \ldots, n}$.

Suppose that $F$ is a primitive of $f$.

If $C$ has start point $z$ and end point $w$, then:

$\ds \int_C \map f z \rd z = \map F w - \map F z$

If $C$ is a closed contour, then:

$\ds \oint_C \map f z \rd z = 0$

## Proof

 $\ds \int_C \map f z$ $=$ $\ds \sum_{k \mathop = 1}^n \int_{a_k}^{b_k} \map f {\map {\gamma_k} t} \map {\gamma_k'} t \rd t$ Definition of Complex Contour Integral $\ds$ $=$ $\ds \sum_{k \mathop = 1}^n \int_{a_k}^{b_k} \paren {\dfrac \rd {\rd t} \map F {\map {\gamma_k} t} } \rd t$ Derivative of Complex Composite Function $\ds$ $=$ $\ds \sum_{k \mathop = 1}^n \paren {\map F {\map {\gamma_k} {b_k} } - \map F {\map {\gamma_k} {a_k} } }$ Fundamental Theorem of Calculus for Complex Riemann Integrals $\ds$ $=$ $\ds \map F {\map {\gamma_n} {b_n} } - \map F {\map {\gamma_1} {a_1} }$ the sum is telescoping $\ds$ $=$ $\ds \map F w - \map F z$ Definition of Endpoints of Contour (Complex Plane)

If $C$ is a closed contour, then $z = w$.

It follows that:

$\map F w - \map F z = 0$

$\blacksquare$