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Propagators in Quantum Chemistry, 2nd Edition by Yngve Öhrn, Jan Linderberg

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Appendix A

Complex Calculus Primer

A complex variable z has a real part x and an imaginary part y and is written z = x + iy. Functions f(z) of a complex variable are our concern. Explicit examples are 1/z, z2, and exp(z). Any function of a complex variable can be expressed as

Image

where u and v are real functions.

Image

Figure A.l: Complex number plane.

A.1   Continuity

A function f(z) is said to be continuous at z = z0 if given any positive number Image, one can find a number δ such that |f(z) − f (z0)| < Image provided that |zz0| < δ.

Let z0 = x0 + iy0. It then follows that |u (x, y) − u (x0, y0)| ≤ |f(z) − f(z0)| < Image if |zz0| < δ i.e., |xx0| < δImage, yy0| < δ/Image. Hence, if f(z) is continuous, so is u(x, y) and similarly v(x, y). Conversely, it can be shown that if u(x, y) and v(x, y) are continuous, so ...

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