In mathematics and physics, especially the study of mechanics and fluid dynamics, the d'Alembert-Euler condition is a requirement that the streaklines of a flow are irrotational.
Let x = x(X,t) be the coordinates of the point x into which X is carried at time t by a (fluid) flow.
Then the d'Alembert-Euler condition is: The d'Alembert-Euler condition is named for Jean le Rond d'Alembert and Leonhard Euler who independently first described its use in the mid-18th century.
It is not to be confused with the Cauchy–Riemann conditions.
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