Spin-Coefficient Formalism  

NP Formalism (Newman-Penrose) > s.a. Edth Operator; gravitational instanton; tetrads.
* Idea: A formalism that recasts the non-linear, second-order Einstein equation as a set of first-order, mostly linear equations for a set of spin connection coefficients, by a systematic use of null tetrads.
* Advantages: It makes transparent the Petrov type of the space, and the Bianchi identities become particularly simple and tractable.
* Applications: It has become a standard tool for finding exact solutions, solving wave equations in curved backgrounds, studying black-hole perturbations (see Chandrasekhar's book), and analyzing the asymptotic structure of gravitational fields at null infinity.
@ General references: Newman & Penrose JMP(62), PRL(65); Pirani in(65); in Misner et al 73, 870–871; Penrose & Rindler 84, 86; in Wald 84, §13.2; Law JGP(09)-a0802 [4D neutral metrics]; Bäckdahl CQG(09)-a0905 [constants in terms of Geroch-Hansen multipole moments]; Nerozzi a1109 [new approach, in transverse tetrads]; Papadopoulos a1707 [finding integrals and identities]; Gómez & Quiroga a1711 [rev]; Barnich et al a1910 [and the BMS current algebra].
@ For Riemannian metrics: Goldblatt GRG(94).
@ And Lanczos potential: Andersson & Edgar JMP(00)gq/98.
@ And Sparling forms: Frauendiener GRG(90).
@ Invariants of Riemann spinor: Haddow GRG(96)gq/95.
@ For 3D Riemannian manifolds: Aazami JGP-a1410 [and results on hypersurface-orthogonal vector fields and curvature].

Individual Coefficients > s.a. weyl tensor.
* Idea: 12 complex quantities that replace the 24 real Ricci rotation coefficients of an orthonormal tetrad when one uses a null one; In an asymptotically flat space time, there is a natural choice of tetrad in the asympototic region for which the Weyl scalars fall off at different powers of r.
Ψ0:= Cabcd la mb lc md, Transverse radiation propagating along la.
Ψ1:= Cabcd la mb lc nd, Longitudinal radiation propagating along la; Pure gauge.
Ψ2:= Cabcd la mb m*c nd, "Mass aspect", the Coulomb part of the field.
Ψ3:= Cabcd la nb m*c nd, Longitudinal radiation propagating along na; Pure gauge.
Ψ4:= Cabcd m*a nb m*c nd, Transverse radiation propagating along na; Falls off most slowly.

GHP Formalism (Geroch, Held & Penrose)
* Idea: A variation of the Newman-Penrose spin-coefficient formalism.
@ General references: Geroch, Held & Penrose JMP(73); Edgar & Ludwig GRG(96), GRG(97), GRG(97)gq [integration], GRG(00); Held GRG(99).
@ Related topics: Ludwig & Edgar CQG(00) [generalized Lie derivative]; Carminati & Vu GRG(01), GRG(03) [Maple package].

Applications and Variations > s.a. horizons; perturbations of FLRW models.
@ Special spacetimes: Bruni et al AIP(05)gq/04 [in astrophysical relativity]; Wu & Shang CQG(07) [stationary]; Zhang et al PRD(09) [stationary electrovacuum]; > s.a. kerr-newman solutions.
@ Higher dimensions: Ortaggio et al CQG(07)gq [Ricci identities]; Durkee et al CQG(10)-a1002 [GHP formalism]; García-Parrado Gómez-Lobo & Martín-García JMP(09), JPCS(11)-a1102 [5D]; Ortaggio et al CQG(13)-a1211 [rev].
@ Variations: Law JGP(09) [four-dimensional neutral metrics].

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