Favard operators

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

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

where ${\displaystyle x\in \mathbb {R} }$, ${\displaystyle n\in \mathbb {N} }$. They are named after Jean Favard.

A common generalization is:

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

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

• Favard, Jean (1944). "Sur les multiplicateurs d'interpolation". Journal de Mathématiques Pures et Appliquées (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).