From Online Dictionary of Crystallography

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Quasi-periodicité (Fr). Quasi-periodicità (It). 準周期性 (Ja). 


A function is called quasiperiodic if its Fourier transform consists of δ-peaks on positions

 k~=~\sum_{i=1}^n h_i  a_i^*,~~({\rm integers ~}h_i)

for basis vectors ai* in a space of dimension m. If the basis vectors form a basis for the space (n equal to the space dimension, and linearly independent basis vectors over the real numbers) then the function is lattice periodic. If n is larger than the space dimension, then the function is aperiodic.


Sometimes the definition includes that the function is not lattice periodic.

A quasiperiodic function may be expressed in a convergent trigonometric series:

f(r)~=~\sum_k A(k) \cos [ 2\pi  k. r+\varphi (k) ].

It is a special case of an almost periodic function. An almost periodic function is a function f(r) such that for every small number ε there is a translation a such that the difference between the function and the function shifted over a is smaller than the chosen quantity:

| f(r+ a)-f( r) |~<~ \varepsilon~~{\rm for ~all~ r} .

A quasiperiodic function is always an almost periodic function, but the converse is not true.

The theory of almost-periodic functions goes back to the work by H. Bohr.