Past exam of the mathematics course of the University of Cambridge / 2010 / iii / Paper 15 / 2 / c

The curvature form of a connection is the endomorphism-valued differential two-form defined byIt is alternating in $X,Y$. To see its tensoriality, replacing $X$ by $fX$ produces an extra $-Y(f){\nabla }_{X}s$ from the second term and an extra $+Y(f){\nabla }_{X}s$ from $[fX,Y]=f[X,Y]-Y(f)X$, so these cancel. Alternation gives linearity over smooth functions in $Y$ too. Replacing $s$ by $fs$ produces the additional coefficient $(X(Yf)-Y(Xf)-[X,Y]f)s=0$; all terms involving one derivative of $s$ also cancel. Thus $R^{\nabla }$ belongs to ${\Omega }^2(M;\mathrm{End}E)$.

In the coefficient-column convention of part (b), expanding the definition givesThe first three terms are $d\Omega (X,Y)$, and the last two are $(\Omega \wedge \Omega )(X,Y)$. This proves the Cartan curvature matrix equation $F=d\Omega +\Omega \wedge \Omega$, with matrix multiplication combined with the wedge product of differential forms.

The pullback connection has matrix ${\Omega }^{\prime }={\varphi }^{\ast }\Omega$. Since the pullback of a differential form commutes with the exterior derivative and the wedge product of differential forms, its curvature form of a connection has matrixChanges of frame of a vector bundle conjugate both sides by the same pulled-back transition matrix, so this identity is intrinsic. In fibre notation the answer isBoth sides act on the same fibre $E_{\varphi (p)}$. No claim that $d\varphi$ preserves Lie brackets of vector fields for an arbitrary map is needed.