|  Bel and Bel-Robinson Tensors | 
Bel Tensor
  $ Def: The tensor, constructed
    from the Riemann tensor, its left and right duals, and its double dual,
Tabcd := \(1\over2\)(Ramcn \(R_b{}^m{}_d{}^n\) + *Ramcn *\(R_b{}^m{}_d{}^n\) + R*amcn \(R^*{}_b{}^m{}_d{}^n\) + *R*amcn *\(R^*{}_b{}^m{}_d{}^n\)) .
Bel-Robinson Tensor
  > s.a. Chevreton Tensor; gowdy spacetime;
  gravitational energy-momentum; stress-energy pseudotensors;
  types of singularities.
  $ Def: The tensor, constructed
    from the Riemann tensor (or Weyl tensor, in the vacuum case),
Tabcd := Ramcn \(R_b{}^m{}_d{}^n\) + *Ramcn *\(R_b{}^m{}_d{}^n\)
= Ramcn \(R_b{}^m{}_d{}^n\) + \(1\over4\)εampq \(\epsilon_b{}^{mr}{}_s\) Rpqcn \(R_r{}^s{}_d{}^n\)
= Ramcn \(R_b{}^m{}_d{}^n\) − \(3\over2\)ga[b Rpq]cn \(R^{\,pq}{}_d{}^n\) .
  * Properties: It is totally
    symmetric, Tabcd
    = T(abcd), traceless,
    T aacd = 0,
    and conserved in vacuum, ∇a
    Tabcd = 0; When it is contracted
    with four future-pointing vectors one always obtains a non-negative value.
  * Spinorial expression: In vacuum
    it can be expressed as Tabcd
    = ψABCD
    ψ'A'B'C'D' .
  * Applications: Used
    formally to construct an "energy" to control the evolution
    of certain Sobolev norms in general relativity; More physically, it
    appears in differences between the gravitational energy-momentum
    calculated in different coordinates and is interpreted as the
    effective super-energy-momentum tensor of free gravitational fields.
  @ References: Ferrando & Sáez GRG(09)-a0807 [algebraic types];
    So a0901 [alternative tensor],
    a1006 [tensorial proof of symmetry].
  @ And quasilocal energy: Brown et al PRD(99)gq/98;
    So a1006 [non-negative, alternative to Bel-Robinson tensor];
    García-Parrado CQG(14)-a1308 [geometric identity and conservation].
  @ Applications: Hacyan a1612 [and gravitational radiation].
  @ Generalizations: Deser in(87),
    gq/99-conf;
    Deser & Franklin CQG(11)-a1011 [for topologically massive gravity],
    CQG(11)-a1108 [none for quadratic curvature theories];
    So a1812 [modification].
  > Online resources:
    see Wikipedia page.
References
  @ Reviews: Douglas GRG(03).
  @ General: Bel CRAS(59);
    Penrose AP(60);
    in Penrose & Rindler 86;
    Robinson CQG(97);
    Bergqvist JMP(98) [positivity];
    Senovilla CQG(00)gq/99;
    Bergqvist & Lankinen CQG(04)gq [characterization].
  @ Interpretation: Garecki CQG(85),
    AdP(01)gq/00;
    Bonilla & Senovilla GRG(97);
    Bergqvist GRG(98);
    García-Parrado CQG(08)-a0707.
  @ Related topics: Deser et al CQG(99)gq [graviton-graviton scattering];
    Douglas GRG(99) [eigentensors];
    Lazkoz et al gq/01-proc [currents];
    Saha et al MPLA(06)gq/05 [and Bianchi I evolution].
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