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The Gauss Bonnet theorem for a closed surface states
$\int_M K = 2 \pi \chi$
where $\chi$ is the Euler characteristic and $K$ is the Gaussian curvature
$K = k_1 k_2$,
which is the multiplication of the principal curvatures, meaning the minimum and maximum curvature, of the surface at the point of evaluation. Is there an intuitive way to understand why the Gauss Bonnet theorem requires the use of the Gaussian curvature, instead of a combination of any other curvatures on the surface?
For example, if I subdivide a surface into square-plaquettes, can I define a topological number using the curvature along the sides of the plaquettes instead?
I simply do not understand where the principal curvatures come from and whether they are avoidable, ie, whether there are alternative formulations of the Gauss Bonnet theorem that do not rely on them.

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Jul 2, 2023 at 14:14
  • $\begingroup$ This is not really a question in differential topology. You should modify the tags appropriately, by including differential geometry for example. $\endgroup$ Commented Jul 2, 2023 at 14:49
  • $\begingroup$ What's more, by Gauss's Theorema Egregium, the Gaussian curvature in fact depends only on the first fundamental form (the Riemannian metric) and can be defined without having the surface sitting in $\Bbb R^3$ and using its extrinsic geometry. $\endgroup$ Commented Jul 2, 2023 at 19:45

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The historical definition of the Gaussian curvature at a point $p$ of a surface in $\mathbb R^3$ was in terms of the product of the minimum and maximum curvatures, etc. However, a more conceptual definition would be to define the Gaussian curvature as the determinant of the Weingarten map $W_p$, also known as the shape operator. Here $W_p$ is an endomorphism of the tangent plane to the surface at the point $p$. The determinant, unlike the trace, is an intrinsic invariant of the Riemannian metric (i.e., the first fundamental form) on the surface. The reason that the Gaussian curvature occurs in the Gauss-Bonnet theorem rather than some other combination is because the Gaussian curvature is the only intrinsic invariant at a point, to begin with.

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