... Ansoldi¹

Email: ansoldi@trieste.infn.it; Webpage: http://www-dft.ts.infn.it/$\sim$ansoldi

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... (infinitesimally)²

Far from being only an idealization of more realistic situations, shells have been extensively used to build concrete models of many astrophysical and cosmological scenarios (for a detailed bibliography please see the references in [Ansoldi(2002)]).

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... conditions³

Conventions are as in [Misner et al.(1970)] and the definition of the (quantities relevant to the concept of) extrinsic curvature can be found in [Barrabes and Israel(1991),Misner et al.(1970)].

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... observer⁴

As usual an overdot, ``$\dot{\quad}$'',denotes a total derivative with respect to $\tau$.

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... Lagrangian⁵

For a more general and deeper discussion see [Hajicek and Kijowski(2000)] and references therein or also, in addition, [Gladush(2001),Sindoni(2004)].

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