## Even and Odd Functions

Some of the Fourier theorems can be succinctly expressed in terms of even and odd symmetries.

Definition: A function is said to be even if .

An even function is also symmetric, but the term symmetric applies also to functions symmetric about a point other than 0.

Definition: A function is said to be odd if .

An odd function is also called antisymmetric.

Note that every finite odd function must satisfy .7.11 Moreover, for any with even, we also have since ; that is, and index the same point when is even.

Theorem: Every function can be decomposed into a sum of its even part and odd part , where

Proof: In the above definitions, is even and is odd by construction. Summing, we have

Theorem: The product of even functions is even, the product of odd functions is even, and the product of an even times an odd function is odd.

Since even times even is even, odd times odd is even, and even times odd is odd, we can think of even as and odd as :

Example: , , is an even signal since .

Example: is an odd signal since .

Example: is an odd signal (even times odd).

Example: is an even signal (odd times odd).

Theorem: The sum of all the samples of an odd signal in is zero.

Proof: This is readily shown by writing the sum as , where the last term only occurs when is even. Each term so written is zero for an odd signal .

Example: For all DFT sinusoidal frequencies ,

More generally,

for any even signal and odd signal in . In terms of inner products5.9), we may say that the even part of every real signal is orthogonal to its odd part:

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