A Modified Variational Principle in Relativistic Hydrodynamics. I.Commutativity and Noncommutativity of the Variation Operator with the Partial Derivative

A model is proposed, according to which the metric tensor field in the standard gravitational Lagrangian is decomposed into a projection (generally - with a non-zero covariant derivative) tensor field, orthogonal to an arbitrary 4-vector field and a tensor part along the same vector field. A theorem...

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