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The Fourier generalized convolutions on time scales \(h\mathbb{N}^0\) and their applications. (English) Zbl 07751558

By restricting the generalized Fourier transform and convolution on time scales \(h\mathbb{Z}\), \(h>0\) to even and odd functions \(f: h\mathbb{N}^0\to\mathbb{C}\), \(\mathbb{N}^0=\mathbb{N}\cup\{0\}\), we can define the \(h\)-Fourier sine and cosine transforms \(F_s\{f\}\) and \(F_c\{f\}\) of \(f\) as follows. \(F_s\{f\}(\omega)=2h\sum_{m=1}^\infty f(nh)\sin(\omega nh)\) for \(\omega\in[0,\frac{\pi}{h}]\), and \(F_c\{f\}\) is given by replacing \(\sin(\omega nh)\) by \(\cos(\omega nh)\) and adding \(hf(0)\). Moreover, if we define the \(h\)-Fourier sine generalized convolution of \(f*g\) on \(h\mathbb{N}^0\) by \[(f*g)(t)=h\sum_{m=1}^\infty f(mh)\big(g(\vert t-mh\vert)-g(t+mh)\big),\] then \(F_s\{f*g\}(\omega)=F_s\{f\}(\omega)F_c\{g\}(\omega)\) provided that \(f, g\in\ell_1(h\mathbb{N}^0)\) and \(f(0)=0\). As an application, a unique solution of a discrete version of the Toepliz-Hankel type integral equation \(x*k=z\) on \(h\mathbb{N}^0\) is given by \(z*u\), where \(u\) is the inverse \(u\) of \(1/F_c\{k\}\), provided that \(k, z\in\ell_1(h\mathbb{N}^0)\), \(F_c\{k\}(\omega)\neq0\) for all \(\omega\in[0,\frac{\pi}{h}]\), and \(u\in\ell_1(h\mathbb{N}^0)\). Similarly, a unique solution of the equation with \(x(nh)\) added to the left hand side is given.

MSC:

42A38 Fourier and Fourier-Stieltjes transforms and other transforms of Fourier type
42A85 Convolution, factorization for one variable harmonic analysis
65T50 Numerical methods for discrete and fast Fourier transforms
44A35 Convolution as an integral transform
45E10 Integral equations of the convolution type (Abel, Picard, Toeplitz and Wiener-Hopf type)
26E70 Real analysis on time scales or measure chains