Linear
and Affine Connections |

**On the Bundle of (Orthonormal) Frames** > s.a. metric [examples];
tetrads [calculation of curvature].

$ __Linear connection__:
A linear connection on a smooth manifold *X* is a
connection on the principal fiber bundle F(*X*) of frames on *X*.

$ __Levi-Civita, metric or
Riemannian connection__: Given the reduced bundle O(*M*) over *M* (i.e.,
a Riemannian metric *g*), it is the only connection such that (i) Γ has
no torsion, Γ^{m}_{ab}
= Γ^{m}_{ba},
and (ii) Parallel transport preserves the metric, *g*_{ab;m} = 0.

$ __Riemann-Cartan connection__: The 1-form

*ω*_{i}^{j} :=
Γ_{i}^{j} – *K*_{i}^{j} ,

where Γ is the Levi-Civita connection 1-form, and *K* the contorsion, with
*T*^{ i} = *θ*^{ j}∧ *K*_{j}^{i}.

* __Cartan structure equations__: They describe the affine connection on
a manifold through the torsion and curvature tensors (which in turn can be obtained
from the connection); If *T* is the torsion 2-form, *θ* the
coframe, Ω the curvature 2-form, and *ω* the connection,

d*θ* + *ω* ∧ *θ* = *T* , Ω =
d*ω* + *ω* ∧ *ω* .

> __Online resources__: For affine connections, see Wikipedia page and Encyclopedia of Mathematics page; for Levi-Civita connection, see Wikipedia page.

**Christoffel Symbols**

$ __First kind__: The Christoffel
symbols of the first kind are defined as {*ab*, *c*}
= \(1\over2\)(∂_{b}*g*_{ac}
+ ∂_{a}*g*_{bc} –
∂_{c}*g*_{ab}).

$ __Second kind__: The Christoffel
symbols of the second kind are defined as {*a* \atop *bc*}
= *g*^{ad} {*bc*, *d*}.

* __And connection__: Given
a choice of coordinates, the components of the
linear connection compatible with a metric *g*_{ab} are
expressed by

Γ^{a}_{bc} =
\(\{{a \atop bc}\}\) = \(1\over2\)*g*^{mn} (*g*_{nb,a}
+ *g*_{an,b} – *g*_{ab.n})
.

* __Gauge transformation__: For
an infinitesimal diffeomorphism generated by *v*^{a}, δΓ^{m}_{ab}
= ∇_{a}∇_{b}*v*^{m} – *R*^{m}_{(ab)c }*v*^{c}.

**On the Tangent Bundle** > s.a. coordinates [Fermi transport]; torsion.

* __History__: Linear connections were invented by Cartan to geometrize
Newtonian gravitation.

* __Existence__: They always exist on a manifold with a countable basis.

* __Parallel transport__: To
define one, start with *x* ∈ *M*, a curve
*γ* through *x*, and a lift of *γ* to
the frame bundle, *γ*_{F(M)};
Fix a vector *X* ∈ *T*_{x}(*M*);
Then the parallel transport of *X* is

*γ*_{T(M)}(*t*):=
*γ*_{F(M)}(*t*)
*X*.

* __Covariant derivative__:
Defined in T(*M*) of *Z* = *γ*_{T(M)}(*t*)
= *b*^{m} (∂/∂*x*^{m})
with respect to *Y* = *c*^{m} (∂/∂*x*^{m})
as

∇_{Y }*Z*:=
(d/d*t*)*γ*_{T(M)}
= (d*x*^{m}(*t*)/d*t*) *D*_{m}*γ*_{F(M)}(*t*)
*X* ,

where *D*_{m} is the covariant
derivative in F(*M*); One finds

∇_{Y}* Z* =
*c*^{n} *b*^{m}_{;n} ∂/∂*x*^{m}
, *b*^{m}_{;n}:=
*b*^{m}_{,n} – *A*^{m}_{ns}* b*^{s}, *A*^{m}_{ns}
= Γ^{m}_{ns} .

* __Flat connection__: If
the torsion and curvature both vanish, the connection is called flat, and there
exist local coordinates in which the connection vanishes;
Then, in other coordinates, Γ = *h*^{–1}d*h*.

* __On a Riemannian manifold__: The connection can be written as

Γ^{a}_{bc} =
\(\{{a \atop bc}\}\) + *K*^{a}_{bc} ,

where {*a* \atop *bc*} are the Christoffel symbols, *Q*^{a}_{bc} = *K*^{a}_{[bc]} the
torsion, and *M*^{a}_{bc} = *K*^{a}_{(bc)} the
non-metricity.

* __Weyl vector__: The trace of the non-metricity,
*B*^{a}:= *M*^{a}_{bc} *g*^{bc}.

**Special Types of Metrics** > s.a. FLRW
models;
spherical symmetry; schwarzschild
spacetime;
symmetry.

@ __References__: Armstrong JGP(07) [Ricci-flat, classification]; Guadagnini et al a1710 [3D, flat].

**References** > s.a. Parallel Transport;
torsion.

@ __General__: Levi-Civita RCMP(17).

@ __Connection and metric__: Schmidt CMP(73);
Hall GRG(88);
Edgar JMP(92);
Thompson CQG(93), JGP(96);
Cocos JGP(06)
[condition for symmetric connection to be Levi-Civita]; Atkins mp/06;
Atkins a0804 [connections that are only locally Levi-Civita]; > s.a. formulations of general relativity.

@ __Lorentz connections__: Hall & Lonie JPA(06)gq/05 [4D]; Pereira AIP(12)-a1210 [in general relativity and teleparallel gravity].

@ __Non-metricity__:
Sobreiro & Vasquez Otoya BJP(10)-a0711 [in
gravity theories]; Mol AACA(17)-a1406 [and general relativity]; Casanova et al MPLA(08)-a0712 [and
torsion, Schouten classification]; Foster et al a1612 [constraints from searches for Lorentz violation]; Järv et al a1802 [non-metricity formulation of general relativity].

@ __Non-commutative geometry__: Dubois-Violette
et al LMP(95)ht/94 [quantum
plane]; Madore et al CQG(95)ht/94 [matrix
geometries]; Madore CQG(96)ht/95;
Mourad CQG(95)ht/94;
Sitarz ht/95;
Masson & Serié JMP(05)
[invariant]; > s.a. geodesics.

@ __And field theory__: Barut et al HPA(93);
in DeWitt & Molina-París MPLA(98)ht [Vilkovisky
connection on space of histories].

@ __On use in gravitational theories__: Kofinas a1712.

@ __Other generalizations__: Milani & Shafei Deh Abad LMP(97)ht/96 [quantized
manifolds, geodesics]; Pham MJM(15)-a1408 [higher affine connections].

> __Other generalizations__: see finsler
geometry; Lagrange Spaces [non-linear
connection]; newton-cartan theory [degenerate metrics].

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send feedback and suggestions to bombelli at olemiss.edu – modified
4 feb 2018