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Quasiperiodicity

From Online Dictionary of Crystallography

Revision as of 04:48, 17 February 2015 by MassimoNespolo (talk | contribs) (Lang.)

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

Definition

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

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

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.

Comment

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

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

[math]f( r)~=~\sum_k A(k) \cos \left( 2\pi k. r+\phi ( k) \right). [/math]

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:

[math]| f(r+ a)-f( r) |~\lt ~ \epsilon~~{\rm for ~all~ r} .[/math]

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.