Hamiltonian Function
H(q, p, t) = \sum_{i} \pi_i rac{\partial q_i}{\partial \tau} + L(q, rac{\partial q}{\partial \tau}), where \pi_i are the conjugate momenta and L is the Lagrangian.
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The statement of the theorem
Given a system with Lagrangian , the Hamiltonian function is defined through the Legendre transformation relating generalized coordinates and conjugate momenta : \n\n \n\nWhere the generalized velocities are implicitly determined by the momenta via the relation . Thus, is expressed solely in terms of .
Source: Wikipedia