Positive-instantaneous frequency representation for transient signals has always been a great concern due to its theoretical and practical importance,although the involved concept itself is paradoxical.The desire and practice of uniqueness of such frequency representation(decomposition)raise the related topics in approximation.During approximately the last two decades there has formulated a signal decomposition and reconstruction method rooted in har-monic and complex analysis giving rise to the desired signal representations.The method decomposes any signal into a few basic signals that possess posi-tive instantaneous frequencies.The theory has profound relations to classical mathematics and can be generalized to signals defined in higher dimensional manifolds with vector and matrix values,and in particular,promotes kernel approximation for multi-variate functions.This article mainly serves as a sur-vey.It also gives two important technical proofs of which one for a general convergence result(Theorem 3.4),and the other for necessity of multiple kernel(Lemma 3.7).Expositorily,for a given real-valued signal f one can associate it with a Hardy space function F whose real part coincides with f.Such function F has the form F=f+iHf,where H stands for the Hilbert transformation of the context.We develop fast converging expansions of F in orthogonal terms of the form F=∞Σk=1ckBk,where Bk's are also Hardy space functions but with the additional properties Bk(t)=ρk(t)eiθk(t),ρk≥0,θ'k(t)≥0,a.e.The original real-valued function f is accordingly expanded f=∞Σk=1 ρk(t)cos θk(t)which,besides the properties of ρk and θk given above,also satisfies H(ρk cos θk)(t)=ρk(t)sinθk(t).Real-valued functions f(t)=ρ(t)cos θ(t)that satisfy the conditionρ≥0,θ'(t)≥0,H(ρ cos θ)(t)=ρ(t)sin θ(t)are called mono-components.If f is a mono-component,then the phase deriva-tive θ'(t)is defined to be instantaneous frequency of f.The above described positive-instantaneous frequency expansion is a generalization of the Fourier se-ries expansion.Mono-components are crucial to understand the concept instan-taneous frequency.We will present several most important mono-component function classes.Decompositions of signals into mono-components are called adaptive Fourier decompositions(AFDs).We note that some scopes of the stud-ies on the 1D mono-components and AFDs can be extended to vector-valued or even matrix-valued signals defined on higher dimensional manifolds.We finally provide an account of related studies in pure and applied mathematics.

Tao QIAN

Macao Center for Mathematical Science,Macao University of Science and Technology,Macao,China

中国数学前沿

[1]Tao QIAN-.Positive-instantaneous frequency and approximation)[J].中国数学前沿,2022(03):337-371

Expositorily,f+iHf,1ckBk,Bk,instan,Decompositions,AFDs

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