Favard operator

In functional analysis, a branch of mathematics, the Favard operators are defined by:

[{\mathcal {F}}_{n}(f)](x)={\frac {{\sqrt {n}}}{n{\sqrt {c\pi }}}}\sum _{{k=-\infty }}^{\infty }{\exp {\left({{\frac {-n}{c}}{\left({{\frac {k}{n}}-x}\right)}^{2}}\right)}f\left({\frac {k}{n}}\right)}

where $x\in \mathbb {R}$ , $n\in \mathbb {N}$ , and $c\in {\mathbb {R^{{+}}}}$ .^[1] They are named after Jean Favard.

Generalizations

A common generalization is:

[{\mathcal {F}}_{n}(f)](x)={\frac {1}{n\gamma _{n}{\sqrt {2\pi }}}}\sum _{{k=-\infty }}^{\infty }{\exp {\left({{\frac {-1}{2\gamma _{n}^{2}}}{\left({{\frac {k}{n}}-x}\right)}^{2}}\right)}f\left({\frac {k}{n}}\right)}

where $(\gamma _{n})_{{n=1}}^{\infty }$ is a positive sequence that converges to 0.^[1] This reduces to the classical Favard operators when $\gamma _{n}^{2}=c/(2n)$ .

References

Favard, Jean (1944). "Sur les multiplicateurs d'interpolation". Journal de Mathematiques Pures et Appliquees (in French). 23 (9): 219–247. This paper also discussed Szász–Mirakyan operators, which is why Favard is sometimes credited with their development (e.g. Favard–Szász operators).

Footnotes

1 2 Nowak, Grzegorz; Aneta Sikorska-Nowak (14 November 2007). "On the generalized Favard–Kantorovich and Favard–Durrmeyer operators in exponential function spaces". Journal of Inequalities and Applications. 2007: 1. doi:10.1155/2007/75142.

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