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In mathematics, the **Favard constant**, also called the **Akhiezer–Krein–Favard constant**, of order *r* is defined as

This constant is named after the French mathematician Jean Favard, and after the Soviet mathematicians Naum Akhiezer and Mark Krein.

This constant is used in solutions of several extremal problems, for example

- Favard's constant is the sharp constant in Jackson's inequality for trigonometric polynomials
- the sharp constants in the Landau–Kolmogorov inequality are expressed via Favard's constants
- Norms of periodic perfect splines.

- Weisstein, Eric W. "Favard Constants".
*MathWorld*.