even and odd functions


Let f be a functionMathworldPlanetmath from to . If f(-x)=f(x) for all x, then f is an even functionMathworldPlanetmath. Similarly, if f(-x)=-f(x) for all x, then f is an odd function.

Although this entry is mainly concerned with functions , the definition can be generalized to other types of function.


A real function is even if and only if it is symmetric about the y-axis. It is odd if and only if symmetric about the origin.


  1. 1.

    The function f(x)=x is odd.

  2. 2.

    The function f(x)=|x| is even.

  3. 3.

    The sine and cosine functions are odd and even, respectively.


  1. 1.

    The only function that is both even and odd is the function defined by f(x)=0 for all real x.

  2. 2.

    A sum of even functions is even, and a sum of odd functions is odd. In fact, the even functions form a real vector space, as do the odd functions.

  3. 3.

    Every real function can be expressed in a unique way as the sum of an odd function and an even function.

  4. 4.

    From the above it follows that the vector spaceMathworldPlanetmath of real functions is the direct sumMathworldPlanetmath of the vector space of even functions and the vector space of odd functions. See the entry direct sum of even/odd functions (example) (http://planetmath.org/DirectSumOfEvenoddFunctionsExample).)

  5. 5.

    Let f: be a differentiable function.

    1. (a)

      If f is an even function, then the derivativePlanetmathPlanetmath f is an odd function.

    2. (b)

      If f is an odd function, then the derivative f is an even function.

    (For a proof, see the entry derivative of even/odd function (proof) (http://planetmath.org/DerivativeOfEvenoddFunctionProof).)

  6. 6.

    Let f: be a smooth function. Then there exist smooth functions g,h: such that


    for all x. Thus, if f is even, we have f(x)=g(x2), and if f is odd, we have f(x)=xh(x2). ([1], Exercise 1.2)

  7. 7.

    The Fourier transformDlmfMathworldPlanetmath of a real even function is purely real and even. The Fourier transform of a real odd function is purely imaginary and odd.


Title even and odd functions
Canonical name EvenAndOddFunctions
Date of creation 2013-03-22 13:34:19
Last modified on 2013-03-22 13:34:19
Owner yark (2760)
Last modified by yark (2760)
Numerical id 12
Author yark (2760)
Entry type Definition
Classification msc 26A06
Related topic HermitianFunction
Defines even function
Defines odd function