
The Fourier transform is a mathematical operation that decomposes a function into its constituent frequencies.The Fourier transform is an integral transform that maps a function of a real variable (often time) to a function of a complex variable (frequency). It decomposes the signal into an orthogonal basis of harmonic oscillations. The result (spectrum) shows the amplitude and phase of each frequency component. The inverse transform allows reconstructing the original signal. Thanks to the Fast Fourier Transform (FFT), the algorithm works very quickly and is used everywhere in signal processing.
Jean-Baptiste Joseph Fourier in 1807 (published in 1822) in a memoir on heat propagation showed that an arbitrary function can be expanded into a trigonometric series. The idea met resistance from Lagrange and Laplace, who doubted the convergence of the series. Later, Dirichlet (1829) clarified the conditions. In the 20th century, Laurent Schwartz extended the transform to generalized functions (Dirac delta function). Today it is the basis of harmonic analysis.
Mathematically, the kernel e^{-iωt} acts as a 'probe': for each frequency ω, we compute the correlation of the signal with a harmonic oscillation of that frequency. The integral measures the 'similarity'. Thanks to the orthogonality of sinusoids, the decomposition is invertible without loss. The Fast Fourier Transform uses the symmetry of complex roots of unity, reducing the computation of a large matrix to a series of small ones. In quantum mechanics, the wave function in momentum representation is the Fourier transform of the coordinate wave function, and the relation Δx Δp ≥ ħ/2 follows directly from the properties of the FT.
Image compression (JPEG), noise filtering in audio, signal analysis in MRI and radar, digitization of music — everywhere where it is necessary to isolate frequencies or remove interference.
In quantum mechanics, the momentum representation of the wave function is the Fourier transform of its coordinate representation. The Heisenberg uncertainty principle arises from the properties of the Fourier transform: the narrower the signal in time, the wider its spectrum.
| Y | variable | observed response — response variable |
| X | variable | input features — feature vector |
| f | variable | true underlying function — function |
| ε | variable | noise term — noise |
| D | variable | monotonic transformation function — monotonic function |
| Γ₁₂ | variable | Mutual coherence function — mutual coherence function watt per metre squared |
| u | variable | Spatial frequency in x-direction — spatial frequency cycle per metre |
| v | variable | Spatial frequency in y-direction — spatial frequency cycle per metre |
| I | variable | Intensity distribution over source — intensity watt per metre squared |
| l | variable | Direction cosine in x — direction cosine |
| m | variable | Direction cosine in y — direction cosine |
| ∬ | operator | Integration over the source plane — double integral |
| W | variable | Wavelet transform coefficient — wavelet transform coefficient |
| s | variable | Scale parameter — scale parameter |
| τ | variable | Translation parameter — translation parameter second |
| x | variable | Input signal — signal |
| t | variable | Time variable — time second |
| ψ* | variable | Complex conjugate of wavelet function — wavelet function |
| ∫ | operator | Integration over time — integral operator |
| \tilde{\kappa} | variable | Fourier transform of convergence — convergence |
| \tilde{\kappa}_B | variable | Fourier transform of B-mode convergence — b mode convergence |
| \tilde{\gamma}_1 | variable | Fourier transform of first shear component — shear component |
| \tilde{\gamma}_2 | variable | Fourier transform of second shear component — shear component |
| k_1 | variable | First component of wave vector — wave vector component radian per metre |
| k_2 | variable | Second component of wave vector — wave vector component radian per metre |
| k | variable | Magnitude of wave vector — wave number radian per metre |
| f | variable | scalar function value — function value |
| x | variable | input vector — vector argument |
| Q | variable | quadratic coefficient matrix — quadratic coefficient matrix |
| b | variable | linear coefficient vector — linear coefficient vector |
| c | variable | constant term — constant term |
| T | operator | flips a matrix or vector over its diagonal — transpose operator |














