Connection (principal bundle)

A principal connection can be specified by a $G$-equivariant horizontal distribution $H\subset TP$ complementary to the vertical bundle, or equivalently by a Lie-algebra-valued connection form $\mathcal{A}$ that reproduces infinitesimal generators and has $H=\ker \mathcal{A}$.

For a smooth strictly convex rigid body with a smooth unique contact point, a configuration consists of its orientation $R\in SO(3)$ and horizontal position $c_{\parallel }\in {\mathbb{R}}^2$; its height is fixed by contact. Translation makes this a principal bundle over $SO(3)$ with group ${\mathbb{R}}^2$. If $r(R)$ points from its reference point to contact and $\Omega$ is its spatial angular velocity, rolling without slipping requires $\dot{c}+\Omega \times r=0$. The displayed translation-valued principal connection vanishes on precisely these velocities. It is translation-invariant and equals the identity on vertical translation vectors. A prescribed orientation curve consequently has a unique horizontal lift from a given initial position. No-slip alone leaves spin free and does not specify a connection over the plane with orientation as its fiber; an additional no-twist constraint is needed for that familiar spherical model.

A connection on a principal circle bundle has local real one-form representatives $A$ transforming by $A\mapsto A+d\chi$. Its curvature is the globally defined two-form $F=dA$. In unit-charge normalization its holonomy around a closed loop is $e^{i\oint A}$, and curvature periods are integer multiples of $2\pi$ on closed surfaces. A circulation representative is defined modulo $2\pi$ under large gauge transformations.

A gauge transformation is an automorphism of a principal bundle covering the identity on the base and commuting with its right group action. Pullback relates gauge-equivalent principal connections. Locally it acts by the displayed formula; the gauge curvature transforms by conjugation. Gauge functions on overlapping charts satisfy the bundle's transition compatibility. Infinitesimally $g=e^{\epsilon }$ gives $\delta A=D_A\epsilon$.

Pulling a principal connection back along a local section $s$ gives a Lie-algebra-valued one-form on the base. If the section changes to $sg$, the form has the displayed inhomogeneous transformation. Its gauge curvature $F=dA+A\wedge A$ transforms homogeneously as $F^{\prime }=g^{-1}Fg$. The collection of such local forms encodes a global principal connection even when no single section exists globally.

A local principal connection form $A$ and a fiber coordinate $\gamma$ reconstruct the connection on a principal bundle in a local section. Changing that section by $s^{\prime }=su$ changes $\gamma$ to $u^{-1}\gamma$ and $A$ to $u^{-1}Au+u^{-1}du$. Substitution cancels the two terms involving $du$, so the reconstructed forms agree on overlaps. Here $A$ is Lie-algebra-valued and matrix conjugation denotes the adjoint action. Agreement across trivializations differs from the right-action equivariance $R_h^{\ast }\omega ={\mathrm{Ad}}_{h^{-1}}\omega$.

The horizontal distribution of a principal connection is the smooth complement $H$ to the tangent spaces of the group orbits. A tangent vector is horizontal exactly when the connection form $\mathcal{A}$ annihilates it.

On a coordinate trivialization of a principal bundle, the right-invariant vector fields $R_a(\gamma )=T_a\gamma$ satisfy $[R_a,R_b]=-c^d{}_{ab}{R}_d$. With $A=A_i^a{T}_{a}d{x}^i$, the displayed lifts project to the commuting coordinate fields and contract to zero with the principal connection. Taking their Lie brackets gives the local curvature of a principal connection coefficients $F_{ij}={\partial }_i{A}_j-{\partial }_j{A}_i+[A_i,A_j]$. Thus these lifts commute exactly when the connection is flat. Arbitrary base fields need not commute, and flatness alone does not supply a global coordinate frame or remove holonomy of a connection.

A local section $s:U\to P$ is horizontal when $ds(TU)\subseteq H$, equivalently when $s^{\ast }\mathcal{A}=0$. A flat principal connection has horizontal sections locally, while its holonomy can obstruct a global horizontal section.

The holonomy of a connection along a closed curve is the group element relating the endpoints of its horizontal lift. A flat connection can have nontrivial holonomy around a noncontractible loop.

An $SU(2)$ connection whose holonomy lies in a maximal circle, equivalently one preserving a splitting $E=L\oplus L^{-1}$. If its holonomy is the entire circle, parallel adjoint sections are exactly the one-real-dimensional diagonal Lie algebra. The off-diagonal real plane is the underlying real bundle of $L^2$ and has circle weight two.

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In the round real projective plane, project the great-circle arc $(\cos t,\sin t,0)$ for $0\le t\le \pi$. Its endpoint and velocity match after antipodal identification, so it is a closed geodesic. On the sphere its tangent and the constant vector $(0,0,1)$ are parallel. Endpoint identification fixes the returned tangent and reverses the returned transverse vector. Its parallel transport is therefore a reflection. Nonorientability permits determinant-minus-one holonomy.

For the Riemannian metric $d{r}^2+f(r{)}^{2}d{\varphi }^2$, where $f>0$ and $\varphi$ has period $2\pi$, parallel transport around $r=r_0$ acts on the orthonormal basis components of a tangent vector by a rotation through $-2\pi f^{\prime }(r_0)$. Indeed, those components obey $u^{\prime }=f^{\prime }(r_0)v$ and $v^{\prime }=-f^{\prime }(r_0)u$ as functions of $\varphi$. Thus all tangent vectors return to themselves exactly when $f^{\prime }(r_0)\in \mathbb{Z}$. The integer criterion concerns holonomy and is stronger than local flatness.

On a connected regular radial interval, trivial holonomy around circular fibres forces the continuous function $f^{\prime }$ to be integer-valued, hence constant. The resulting Riemannian metrics have $f(r)=nr+b$, with $n\in \mathbb{Z}$ and $f>0$ on the interval. Their Gaussian curvature is $-f^{\prime \prime }/f=0$. For $n\ne 0$, $\rho =f/|n|$ and $\theta =|n|\varphi$ give a local isometry to the Euclidean plane; for $n=0$, $r,b\varphi$ give a flat cylindrical chart. The polar developing map is injective on the regular domain when $|n|=1$, and is a multiple covering when $|n|>1$. Trivial holonomy therefore does not imply a global Euclidean chart. The slope is a discrete parameter, not an unrestricted real one.

For a connected Riemannian manifold, the full holonomy group consists of parallel transport maps around all piecewise smooth loops based at $p$. Metric compatibility makes them orthogonal. Basepoint changes conjugate this group by parallel transport. This is full holonomy, not just the subgroup using contractible loops.

The holonomy representation is the natural action of the Riemannian holonomy group on $T_{p}M$. It induces representations on cotangent spaces and tensor powers. Irreducibility means that no nonzero proper real linear subspace of $T_{p}M$ is invariant.

A nonzero parallel one-form determines a nonzero parallel vector by metric duality. Its value is fixed by the full Riemannian holonomy group, so its span is an invariant line. This contradicts irreducibility when $\dim M\ge 2$. The dimension condition matters: the standard circle has trivial but irreducible one-dimensional holonomy representation and a nonzero parallel one-form.

Evaluation at a point identifies parallel tensor fields with tensors fixed by the full Riemannian holonomy group. A parallel field is fixed after transport around every loop. Conversely transport a fixed tensor along paths from the basepoint; loop invariance makes the result independent of the path, and local parallel transport shows smoothness and parallelness. Connectedness gives uniqueness.

For the principal $O(k)$-bundle of orthonormal $k$-frames over the real Grassmannian, the canonical horizontal vectors move every frame vector orthogonally to the spanned plane. If a frame is represented by a matrix $Q$ with $Q^{\mathsf{T}}Q=I$, the connection form is