In mathematics, any integrable function can be made into a periodic function with period *P* by summing the translations of the function by integer multiples of *P*. This is called **periodic summation:**

When is alternatively represented as a Fourier series, the Fourier coefficients are equal to the values of the continuous Fourier transform, at intervals of .^{[1]}^{[2]} That identity is a form of the Poisson summation formula. Similarly, a Fourier series whose coefficients are samples of at constant intervals (*T*) is equivalent to a **periodic summation** of which is known as a discrete-time Fourier transform.

The periodic summation of a Dirac delta function is the Dirac comb. Likewise, the periodic summation of an integrable function is its convolution with the Dirac comb.

If a periodic function is instead represented using the quotient space domain then one can write:

The arguments of are equivalence classes of real numbers that share the same fractional part when divided by .