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In mathematics, a doubly periodic function is a function defined on the complex plane and having two "periods", which are complex numbers u and v that are linearly independent as vectors over the field of real numbers. That u and v are periods of a function ƒ means that
for all values of the complex number z.
The doubly periodic function is thus a two-dimensional extension of the simpler singly periodic function, which repeats itself in a single dimension. Familiar examples of functions with a single period on the real number line include the trigonometric functions like cosine and sine. In the complex plane the exponential function e is a singly periodic function, with period 2πi.
As an arbitrary mapping from pairs of reals to reals, a doubly periodic function can be constructed with little effort. For example, assume that the periods are 1 and i, so that the repeating lattice is the set of unit squares with vertices at the Gaussian integers. Values in the prototype square can be assigned rather arbitrarily and then 'copied' to adjacent squares. This function will then be necessarily doubly periodic.