|  Killing Tensor Fields and Killing Forms | 
In General
  > s.a. killing vector fields / conservation laws.
  $ Killing tensor: An n-th rank
    Killing tensor is a symmetric covariant tensor, Kab...
    c = K(ab... c), such that
    ∇(m Kab...
    c) = 0.
  $ Killing form: A differential
    form ωab... c
    = ω[ab...
    c] whose covariant derivative is totally skew-symmetric.
  * Conserved quantities:
    While Killing vectors give the linear first integrals of the geodesic equations,
    Killing tensors give the quadratic, cubic, and higher-order first integrals;
    For every Killing tensor Kab... c,
    um ∇m
    (Kab... c
    ua ub
    ··· uc) = 0,
    if ua is tangent to
    affinely parametrized geodesics.
  * Example: The metric itself
    is always a rank-2 Killing tensor; The associated conserved quantity is
    the norm squared gab
    uaub
    = uaua.
  * Applications: Integrability
    of geodesics in Kerr-Newman spacetime.
  @ General references:
    Sommers JMP(73) [and particle constants of motion];
    Dolan et al GRG(89) [significance];
    Benn JMP(06) [and mechanics];
    Coll et al JMP(06)gq [spectral decomposition];
    Garfinkle & Glass CQG(10)-a1003 [method, in spacetimes with symmetries];
    Cariglia et al CQG(14);
    Houri et al CQG(18)-a1704 [integrability].
  @ Special types of manifolds: 
    Rosquist & Uggla JMP(91) [2D spacetimes];
    Smirnov & Yue JMP(04)mp [constant curvature pseudo-Riemannian];
    Belgun et al DG&A(06) [symmetric spaces];
    Cariglia & Galajinsky PLB(15)-a1503 [Ricci-flat spacetimes];
    Vollmer JGP(17)-a1602 [in stationary and axisymmetric spacetimes].
  @ From conformal Killing vectors:
    Koutras CQG(92);
    Barnes et al gq/02-proc;
    Rani et al CQG(03).
  @ Second-rank:
    Walker & Penrose CMP(70) [Kerr spacetime];
    {Bombelli & Rosquist};
    Baleanu gq/98,
    NC-gq/98;
    Chanu et al JMP(06) [2D flat manifold];
    Brink PRD(10);
    Oota & Yasui IJMPA(10)
      [generalized Kerr-NUT-de Sitter spacetime];
    Keane & Tupper CQG(10) [pp-wave spacetimes].
  @ Third-rank: Rosquist & Goliath GRG(98);
    Karlovini & Rosquist GRG(99)gq/98 [1+1 dimensions];
    Baleanu G&C(99).
Killing-Yano Tensors / Forms > s.a. Taub-NUT Metric.
  $ Def: An n-th rank
    Killing-Yano tensor is an n-form ηab...
    c, such that ∇(m
    ηa)b... c = 0.
  * Relationships: A Killing 2-tensor can
    be defined from a Killing-Yano tensor by Kmn:=
    ηmb... c
    ηnb... c.
  * Example: The alternating
    tensor εab... c
    is a Killing-Yano tensor; The corresponding rank-2 Killing tensor is (proportional to) the metric.
  @ General references: Yano AM(52);
    Kastor et al CQG(07)-a0705
      [conditions for graded Lie algebra with respect to the Schouten-Nijenhuis bracket];
    Batista CQG(14)-a1405 [Killing-Yano tensors of order n – 1].
  @ In gravitation:
    Dietz & Rüdiger PRS(81),
    PRS(82);
    Hall IJTP(87);
    Baleanu NCB(99)gq/98 [and Nambu tensors];
    Ferrando & Sáez GRG(03) [Rainich problem];
    Açık et al GRG(10) [and conserved gravitational currents].
  @ Special types of spacetimes:
    Howarth & Collinson GRG(00) [spherical static];
    Jezierski & Lukasik CQG(06) [Kerr];
    Kubizňák a0909-proc [black holes];
    Acik et al JMP(10) [spherically symmetric];
    Houri et al CQG(12) [with torsion, classification];
    Garfinkle & Glass JMP(13)-a1302 [spacetimes admitting a hypersurface-orthogonal Killing vector];
    Frolov et al PRD(18)-a1712;
    > s.a. kerr spacetime.
  @ Conformal Killing-Yano tensors: Jezierski APPB(08)-a0705 [asymptotically AdS];
    Kubizňák & Krtouš PRD(07)-a0707 [for Plebański-Demiański type-D solutions].
Killing Spinors
  * Examples: Manifolds with
    Killing spinors include nearly Kähler 6-manifolds, nearly parallel
    G2-manifolds in dimension 7,
    Sasaki-Einstein manifolds, and 3-Sasakian manifolds.
  @ General references:
    Baum m.DG/02 [conformal];
    Bohle JGP(03) [on Lorentzian manifolds];
    in Cariglia CQG(04)ht/03 [and Yano tensors];
    Harland & Nölle JHEP(12) [instantons on manifolds with Killing spinors];
    Cole & Valiente Kroon CQG(16)-a1601 [implications of their existence];
    Gutowski & Sabra a1905 [in 4D supergravity];
    Cortés et al a1911 [new framework].
  @ Specific types of spacetimes: Van den Bergh CQG(10)-a0908 [spacetimes admitting non-null valence-two Killing spinors];
    Van den Bergh CQG(11) [homogeneous Petrov-type D Killing spinor spacetimes];
    Batista PRD(16)-a1512 [in 6D spacetime].
  > Online resources:
    see Wikipedia page.
Other Variations and Generalizations
  $ Killing form on a Lie algebra:
    Given a Lie algebra \(\cal G\), its Killing form is the bilinear form
    B(X, Y) = tr[Ad(X), Ad(Y)].
  @ Generalized Killing tensors:
    Collinson & Howarth GRG(00);
    Howe & Lindström JHEP(16)-a1511 [in superspace].
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