# pull-back connection

I have a question related to the definition of the pull-back connection, more specifically about its uniqueness or the canonical way to induce it.

The definition that one finds in general goes along the following lines: let $F:P\rightarrow B$ be map between differentiable manifolds, let $\pi:E\rightarrow B$ be a vector bundle and $\nabla$ a connection on $E$. Then the connection $F^*\nabla$ is uniquely determined by the following relation

$(F^*\nabla)_X(F^* s):=F^*(\nabla_{df(X)} s)$.

This should uniquely determine the connection right?

Let us start with the most trivial case, when $B$ is a point. Then $E$ is a vector space and the pull-back of $E$ is a trivial vector space over $P$. A connection on $E\rightarrow pt$ is an endomorphism of $E$ (which trivially satisfies the Leibniz relation). Could anybody explain how does that canonically induce a connection on $P\times E$, presumably the trivial connection $d$ if one starts with the zero endomorphism of $E$? Leibniz relation does not suffice. One could say let us make a convention here. But the more general question is how does one define $(F^*\nabla)_X$ when $X\in\ker{dF}$, in general?

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What is $F^*s$? –  Deane Yang Dec 13 '10 at 16:06
I don't know how to define $F^*\nabla$ functorially (i.e., without using local co-ordinates and/or trivializations). I suggest first working this out using local co-ordinates and trivializations. –  Deane Yang Dec 13 '10 at 16:17
I take it back. Your equation is right. But I still recommend playing around with it using local co-ordinates and trivialization. It's a rather confusing equation (at least for me). –  Deane Yang Dec 13 '10 at 16:26
From the right-hand side of your equation, if $dF(X) = 0$ then $(F^*\nabla)_X = 0$ as well (note it should be $dF$, not $df$). –  Matt Noonan Dec 13 '10 at 16:44
Over a pt, connections vanish (since vector fields & 1-forms are 0). In general "$dF(X)$" makes no sense as a vector field on $B$. View connections as additive maps from sections of $E$ to sections of $E \otimes \Omega^1_B$ over varying opens in $B$. Local sections of $F^{\ast}(E)$ are function-linear combinations of $F$-pullbacks of local sections of $E$, so the pullback rule (using pullback of 1-forms and of local sections) and Leibniz yield uniqueness. Construction with local coords gives local existence (& yields d when $B$ is pt and $E$ trivial), so by uniqueness get global existence. –  BCnrd Dec 13 '10 at 17:15

Here's my summary of the situation:

1) First, observe that the space of local sections of the pullback bundle is generated by the space of sections of the original bundle composed with the map $F$. (This is better stated using sheaf language)

2) So, using the Leibniz rule, it suffices to define the pullback connection on a section obtained by composing a section of the original bundle with $F$.

3) The formula given above accomplishes this. It is worth noting that in this formula you should view $X$ as a single vector and not as a vector field.

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As an aside, I stumbled onto this, because the use of a pullback connection is implicit when analyzing variations of geodesics and Jacobi fields on a Riemannian manifold. I've never seen this discussed explicitly in any textbook or paper, but it's actually quite necessary to make all the arguments rigorous. –  Deane Yang Dec 13 '10 at 19:06
Dear Deane: Aha, I didn't think of the interpretation with $X$ as a single vector (since the notation $\nabla_X$ is usually used with $X$ a vector field). OK, that then makes sense, but working that pointwise then requires a separate argument to verify the smoothness aspect (i.e., recognizing how to bypass the single-vector formulation after all). If we use the 1-form formulation then we work with local smooth sections throughout, so smoothness "comes along for the ride", and the single-vector interpretation can be inferred afterwards. Well, six of one, half dozen of the other. :) –  BCnrd Dec 13 '10 at 19:41
Dear BCnrd, could you clarify what smoothness aspect you are referring to? –  Deane Yang Dec 13 '10 at 20:16
Dear Deane: I just mean that if we make a pointwise construction then we may need to do more work (an explicit calculations over a suitable small open domain, or something else) to verify that its output is smooth in local coordinates, whereas if we make construction in terms of local sections of bundles (or in other words, from the viewpoint of sheaves) then the smoothness of the output of the connection operator drops out automatically from the framework. –  BCnrd Dec 13 '10 at 23:50
Dear BCnrd, thanks for the clarification. But I don't think there's any additional work required to verify smoothness. Smoothness of the pull-back connection follows directly from the smoothness of the original connection, smoothness of $F$, linear pointwise dependence on the tangent vector, and the Leibniz formula. –  Deane Yang Dec 14 '10 at 0:01
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Hi, I have seen the equation you gave as a definition many times. For example, I think it is also used in a corresponding Wikipedia article. Nevertheless, as you correctly pointed out, it does not give a reasonable/unique description. A better formulation/definition of the pullback connection can be found for example in Milnor's and Stasheff's book 'Characteristic classes' on p. 292, Lemma 3 and its proof (definition by universal property/commutative diagram; proof: computation in local coordinates; it's the precise version of what the equation you gave tries to capture). I hope this helps more or less.

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The commutative diagram in Milnor and Stasheff is exactly the same equation, only written in a different language. As others pointed out, it does define the pull-back connection uniquely but you can not use it directly for arbitrary sections of the pull-back bundle. –  Sergei Ivanov Dec 14 '10 at 9:03
To Matt: The relation $(F^*\nabla)_X=0$ does not satisfy Leibniz relation. Meanwhile I found out the answer to my question (a friend clarified it for me).
It turns out that there is an isomorphism $\Gamma(P;\pi^*E)\simeq \Gamma(P;\mathbb{C})\otimes \Gamma(B;E)$ where the tensor product is over $\Gamma(B;\mathbb{C})$. It is obviously true when $E$ is a vector space. Now use Leibniz relation $\nabla_X(f\otimes s)=X(f)\otimes s+f\otimes\nabla_Xs$ to extend the connection from $\Gamma(B;E)$ to $\Gamma (P;\pi^*E)$. In the trivial case one gets indeed that $\nabla_X(f\otimes s)=X(f)\otimes s$.
@Unknown: see my comment above. The equation you wrote down is only meaningful for pull-backs of sections $s$. If you multiply $F^*s$ by an arbitrary function $h$, it will no longer be the pull back of a section over $B$ if $h$ is not constant along $F^{-1}B$. In other words, you were trying to force Leibniz rule somewhere it has no business being. The given expression is, indeed, enough to specify the pull-back connection, but it is not so simple as plug-and-play. –  Willie Wong Dec 13 '10 at 17:15
Willie i didn't say it was easy:) In the background lurks the isomorphism $\Gamma(P;\pi^*E)\simeq \Gamma(P;\mathbb{C})\otimes \Gamma(B;E)$ which is by not obvious(at least not to me). But Leibniz relation does make sense and it does define the connection everywhere and it also explains the trivial connection on the trivial bundle. –  user11538 Dec 13 '10 at 17:29
Dear unknown: the isom. you're using is false in complex-analytic and algebraic cases (unsure in $C^{\infty$-case, but seems silly to use a defn that only works there when there's a simple procedure applicable in all cases). Think more locally (it's good for you!): use local existence and global uniqueness to get global existence. What you propose with "global" tensors works locally with no hard work; then use uniqueness to globalize. Your comment to Matt is unclear since your original statement of the pullback-relation with global vector fields makes no sense ($dF(X)$ makes no sense on $B$). –  BCnrd Dec 13 '10 at 18:26
Dear BCnrd, Thank you for your interesting comments but... 1) I was only interested in the differentiable case. I am not sure what you mean by a connection in the analytic or algebraic cases. (an analytic(algebraic) splitting of the tangent bundle?) so I am really satisfied with the isomorphism of smooth sections i was talking about. By the way, it's not mine. 2)I admit the comment to Matt lacked details, probably thinking that it was clear $X$ should be thought as a tangent vector and not a vector field (as I wrote); (retrospectively that was really not the main issue of the question); –  user11538 Dec 13 '10 at 20:30