|  Petrov, Petrov-Pirani Classification of Spacetimes | 
In General > s.a. solutions of
  general relativity; types of spacetimes.
  * Idea: A classification
    scheme for algebraically special spacetimes, based on properties of the Weyl
    tensor, or more precisely on the structure of the degenerate spinors of
    the Weyl spinor ψABCD
    = oA ιB
    αC
    βD in 4-dimensional Lorentzian manifolds.
  * Applications: Studies of gravitational radiation.
  * Types: Depending on the number of distinct principal null directions,
  - I or {1111}, four distinct principal null directions.
  - II, one doubly degenerate principal null direction,
    or a geodesic shear-free null congruence.
  - D or {2 2}, two doubly degenerate principal null directions.
  - III, one triply degenerate principal null direction.
  - N, one four-fold degenerate principal null direction.
Examples > s.a. Robinson-Trautman Spacetimes.
  * Type D and black holes: All
    black-hole solutions are of Petrov type D, as are radially homothetic spacetimes
    (spherical collapse); The Kerr metric is the only asymptotically flat, stationary,
    axially symmetric, type-D solution of the vacuum Einstein equation; The most important
    solutions for numerical relativity are of type I, I-D, and II, and pp-waves are of type N.
  @ References: Wu & Bai PRD(08) [Kerr spacetime].
References
  @ General: Petrov re GRG(00);
    Petrov; Kalotas & Eliezer AJP(83)jan [elementary];
    Penrose & Rindler 86;
    Yoon ht/92 [1+1 view];
    in Stephani et al 03;
    Zakhary et al GRG(03) [new algorithm];
    & Hauser, & Held;
    Cherubini et al CQG(05)gq/04 [speciality index and BKL map];
    Acevedo & López-Bonilla GRG(05);
    Pravda JPCS(06)gq/05 [rev].
  @ Type I: Edgar IJTP(79);
    McIntosh & Arianrhod CQG(90) [degeneracy];
    Ferrando & Sáez CQG(03)gq,
    JMP(06)gq/03 [aligned Papapetrou fields];
    Wylleman CQG(08)-a0801 [generically asymmetric rotating dust].
  @ Type II: Aksteiner et al a2101 [geometry].
  @ Type D, classification: Collins et al CQG(91) [vacuum, Karlhede classification];
    Collins & d'Inverno CQG(93) [non-vacuum, Karlhede classification];
    Ferrando & Sáez CQG(98) [characterization],
    JMP(04)gq/02 [classification];
    Edgar et al CQG(09)-a0812 [vacuum, identities and classification];
    Coley & Hervik CQG(09) [vacuum].
  @ Type D, other: Penrose GRG(95) [behavior of complex component of Weyl curvature];
    Hodgkinson JMP(01) [embedding class two];
    Cherubini et al CQG(04)gq [perturbations];
    Pravda et al CQG(07)-a0704 [higher dimensions];
    De Groote & Van den Bergh a0812 [pure radiation];
    Wylleman JPCS(10)-a1204 [perfect fluid, with constant zero-order Riemann invariants].
  @ Type III:
    Hacyan & Plebański IJTP(75) [electrovac];
    Nesterov G&C(03) [electrovac with cosmological constant];
    Fuster & van Holten PRD(05) [Einstein-Yang-Mills];
    Ortaggio et al PRD(10) [Einstein spacetimes, types III and N];
    > s.a. huygens principle;  Kundt Waves.
  @ Type N: Nesterov G&C(03) [electrovac with cosmological constant];
    Edgar & Machado CQG(05) [pure radiation];
    > lorentzian geometry [classification].
  @ Results: Wagh & Govinder gq/02 [radially homothetic is type D].
  @ Related topics: Owen PRD(10)-a1004
      [degeneracy measures, "approximate" Petrov classes, numerical black-hole mergers];
    Gath et al a1506 [and holographic reconstruction of spacetime];
    > s.a. Catastrophe Theory; Goldberg-Sachs Theorem.
Other Classifications and Generalizations
  > s.a. Segre Classification.
  @ Riemannian: Hacyan PLA(79);
    Karlhede CQG(86).
  @ Approximate:
    Ellis & McCarthy AP(87).
  @ General tensors and dimensions:
    De Smet CQG(02) [5D];
    Coley et al CQG(04),
    CQG(08) [higher-dimensional];
    Milson et al IJGMP(05)gq/04;
    Coley & Pelavas GRG(06);
    Reall a1105-ch [in higher dimensions];
    Batista GRG(13)-a1204 [4D, all signatures];
    Reall et al CQG(13)-a1211 [5D algebraically-special vacuum spacetimes];
    Batista a1311-PhD [all dimensionalities and signatures].
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