# What is the name of this delta operator

In Euler-Lagrange Equation: $${\delta \over \delta y}F \equiv {\partial F \over \partial y}- {d \over dx} ({\partial F \over \partial {y'}})$$

What is the name of operator $\delta$ here?

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## 1 Answer

It's called the functional or variational derivative with respect to $y$.

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