Constraint Algebra in Bigravity

Understand

The constraint algebra is derived in the second order tetrad Hamiltonian formalism of the bigravity.

• This is done by a straightforward calculation without involving any insights, implicit functions, and Dirac brackets.
• The tetrad approach is the only way to present the bigravity action as a linear functional of lapses and shifts, and the Hassan-Rosen transform (characterized as "a complicated redefinition of the shift variable" according to the authors) appears here not as an ansatz but as fixing of a Lagrange multiplier.
• A comparison of this approach with the other ones is provided.