Diffeomorphisms |

**In General**
> s.a. Hole Argument; Homeotopy Group.

$ __Def__: A diffeomorphism is
a bijection *f* : *M* to *N* between differentiable
manifolds *M* and *N* under which the differentiable structure
is invariant.

* __Effect on geometric quantities__:
In general, under an infinitesimal diffeomorphism generated by a vector field
*v*, δ*T* = \(\cal L\)_{v}
*T*; For a metric and a connection

δ*g*_{ab}
= 2 ∇_{(a} *v*_{b)}
, δΓ^{m}_{ab}
= ∇_{a}
∇_{b} *v*^{m}
− *R*^{m}_{(ab) c}
*v*^{c} .

* __Descriptors__: Given an infinitesimal
diffeomorphism of a manifold, *x*^{m}
\(\mapsto\) *x*^{m}
+ *ξ*^{m}(*x*),
the descriptors are the functions *ξ*^{m}(*x*),
i.e., the vector field generating the diffeomorphism [@ in Anderson 67].

@ __Diffeomorphism groups__: in Marsden 74;
Michor 80;
Banyaga 97;
> s.a. types of groups [mapping class].

@ __Representations__: Goldin et al JPA(83) [of \(\mathbb R\)^{3}];
Maxera ht/98 [non-linear realizations];
Larsson gq/99 [of extended algebra].

@ __Geometry__: Khesin & Misiolek JMFM(05)m.DG [asymptotic directions];
Gordina & Lescot mp/05
[Ricci curvature of Diff(S^{1})/S^{1}].

@ __Related topics__: Dolgopyat CMP(00) [mostly contracting];
Larsson a1205 [new extensions of diffeomorphism algebras].

**Witt Algebra**
> s.a. Virasoro Algebra.

$ __Def__: The infinite-dimensional
Lie algebra of Diff(S^{1}).

* __Generators__: Given
by *L*_{n}
= −i exp{i*nθ*} ∂/∂*θ*,
with commutation relations [*L*_{m},
*L*_{n}] = (*n*−*m*)
*L*_{m+n}.

@ __References__: Schlichenmaier a1111 [second cohomology].

**Types of Diffeomorphisms and Manifolds**

* __Large diffeomorphims__: The ones
not in the component connected to the identity, which form the *modular group*.

* __3D__: The Smale conjecture
states that the diffeomorphism group Diff(S\(^3\)) has the same
homotopy type as SO(4); In proving the Smale conjecture, A Hatcher
proposed the Generalized Smale conjecture that for all closed 3-manifolds
Diff(\(M^3\)) is homotopic to Isom(\(M^3\)), which has been proven for many
special cases; Don Witt has proposed a correspondence between the Generalized
Smale conjecture and gauge fixing in gauge/diffeomorphism invariant theories
as a procedure for proving it in such situations – the idea is to use
the fact that the diffeomorphism group is the gauge group by which one divides
to obtain the space of gauge-invariant configurations from a space of
gauge-dependent ones, and if both of the latter are known well enough
one can set up an exact sequence with which one calculates all homotopy
groups of the diffeomorphism group.

@ __1D__: Banakh & Yagasaki Top(09)
[Diff(\(\mathbb R\)), topological structure];
Neretin JGM(17)-a1601 [Diff(S^{1}), reproducing kernels and analogs of spherical functions];
Weiner CMP(17)-a1606 [Diff(S^{1}), local equivalence of representations].

@ __2D__: Thurston BAMS(88) [geometry and dynamics].

@ __3D__: Bonatti et al Top(04) [gradient-like];
Benatti & Wilkinson Top(05) [transitive partially hyperbolic].

@ __On compact Riemannian manifolds__: Delanoë DG&A(04) [gradient rearrangement].

@ __ Volume-preserving__: Sato EPJC(14)-a1404 [structure constants];
> s.a. general relativity [origin] and formulations;
modified versions of QED; quantum gauge theories;
spheres.

@ __Generalized__: Dimitrijević & Wess ht/04-talk [deformed bialgebra on non-commutative space];
> s.a. discrete gravity below; non-commutative geometry.

@ __Related topics__: Giacomini PRD(04)ht [Poisson algebra with spacetime bifurcations];
Harvey in Bullett et al 17 [modular group].

**And Physical Theories**
> s.a. canonical general relativity; Covariance [general
covariance]; symmetries; symmetry breaking.

* __Quantum gravity__:
Large diffeomorphisms give rise to theta sectors.

@ __And gauge symmetries__:
Kuchař & Stone CQG(87) [parametrised Maxwell field];
Pons et al PRD(00)gq/99 [in Ashtekar variables],
JMP(00)gq/99 [in Einstein-Yang-Mills];
> s.a. types of gauge theories
and quantum gauge theories.

@ __And general relativity / quantum gravity__:
Chamblin & Gibbons gq/95-proc [spacetime topology and time-orientation reversal];
Aldaya & Jaramillo IJMPA(03)gq/02 [representations of diffeomophism groups];
Larsson in(06)-a0709 [anomalies];
Samanta IJTP(09)-a0708 [Lagrangian gravity];
Dittrich ASL(09)-a0810;
Ashtekar GRG(09)-a0904-in [consequences of diffeomorphism invariance];
Farkas & Martinec JMP(11)-a1002 [extension of spatial diffeomorphisms];
Salisbury et al IJMPA(16)-a1508 [in canonical general relativity, Hamilton-Jacobi approach];
Patrascu JMP(16)-a1410 [extension to topology].

@ __And discrete geometry / gravity__:
Kunzinger & Steinbauer CQG(99) [distributional, pp-wave example];
Bahr & Thiemann CQG(09)-a0711 [distributional, and combinatorial lqg];
Gambini & Pullin CQG(09)-a0807 [emergent diffeomorphism invariance];
Bahr & Dittrich AIP(09)-a0909;
Baratin et al PRD(11)-a1101 [in group field theories];
Wetterich PRD(12)-a1110;
Dittrich & Steinhaus PRD(12)-a1110 [measure and triangulation independence];
Wetterich PRD(12) [on a lattice];
Dittrich a1201-proc;
Bahr et al Sigma(12) [and constraints];
Bonzom & Dittrich CQG(13)-a1304 [discrete hypersurface deformation algebras];
> s.a. lattice gravity; loop quantum gravity.

@ __Large diffeomorphisms__: Giulini BCP(97)gq/95 [and geons],
NPPS(97)gq;
Balachandran & de Queiroz JHEP(11)-a1109 [anomalies and mixed states].

@ __Broken / partial diffeomorphism invariance__: Carballo-Rubio PRD(15)-a1502 [Weyl transverse gravity, and the cosmological constant];
> s.a. Background.

@ __Non-relativistic diffeomorphism invariance__:
Andreev et al PRD(14) [examples and applications];
Banerjee et al PLB(14)-a1404,
IJMPA(17)-a1604 [symmetries].

@ __Related topics__:
Sorkin in(93) [and particle symmetries];
Pons CQG(03)gq [and phase space for generally covariant theories];
Dedushenko JHEP(10)-a1007 [diffeomorphism anomaly in quantum mechanics];
Kleppe & Nielsen a1412-conf [diffeomorphism invariance from Random Dynamics];
Pooley a1506
[diffeomorphism invariance vs background independence];
Kreimer & Yeats MPAG(17)-a1610 [quantum field theories, and renormalization];
Johns a1908
[different interpretations of active diffeomorphisms, and substantivalism].

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