[Math] Why are Lagrangian submanifolds called Lagrangian

classical-mechanicslagrangian-submanifoldssg.symplectic-geometryterminology

Much of the terminology in symplectic geometry comes from classical mechanics: the symplectic manifold is modeled on a cotangent bundle $T^*N$ of some configuration space $N$ with local position coordinates $q_i$ and momentum coordinates $p_i$ such that $\omega = \sum_i dq_i \wedge dp_i$. Then, a Hamiltonian – a.k.a. a smooth "energy" function – on this phase space induces a flow satisfying Hamilton's equations from classical mechanics:
$$\frac{dp_i}{dt} = -\frac{\partial H}{\partial q_i} \hskip{1 in} \frac{dq_i}{dt} = \frac{\partial H}{\partial p_i}$$
In this physical interpretation, what are Lagrangian submanifolds? In particular, why are they named Lagrangian? Is there a relationship between this notion and the Lagrangian formulation of mechanics?

(Note: this question has a lot of great answers providing some physical or geoemtric intuition for Lagrangian submanifolds – in a cotangent bundle, both fibers and the images of closed sections (closed $1$-forms on $N$) are motivating examples of Lagrangian submanifolds – but it does not address the source of the terminology.)

Best Answer

This echos the 2017 comments, but since the question has now been bumped to the front page it might be helpful to give the actual source in Maslov's book [1].

[1] V.P. Maslov, Perturbation Theory and Asymptotic Methods (in Russian, Moscow, 1965); Théorie des pertubations et méthodes asymptotique (French translation, Paris, 1972).


The Lagrangian manifold (variété lagrangienne) is introduced on page 114-115:

114 115 338

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