Favard operator

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

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

where $x\in \mathbb {R}$, $n\in \mathbb {N}$. 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. This reduces to the classical Favard operators when $\gamma _{n}^{2}=1/(2n)$.

References

• 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).