By Luca Capogna, Donatella Danielli, Scott D. Pauls, Jeremy Tyson

This booklet offers an updated account of development on Pansu's celebrated challenge at the sub-Riemannian isoperimetric profile of the Heisenberg staff. It additionally serves as an creation to the overall box of sub-Riemannian geometric research. It develops the tools and instruments of sub-Riemannian differential geometry, nonsmooth research, and geometric degree conception compatible for assaults on Pansu's problem.

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**Extra resources for An Introduction to the Heisenberg Group and the Sub-Riemannian Isoperimetric Problem (Progress in Mathematics)**

**Example text**

Applied models 41 Heisenberg group, is locally approximated by it. We ﬁrst introduce RT and then discuss the approximation by H. The roto-translation group, RT , is the group of Euclidean rotations and translations of the plane equipped with a particular sub-Riemannian metric. More precisely, RT is a three-dimensional topological manifold diﬀeomorphic to R2 × S1 with coordinates (x, y, θ). We identify the vector ﬁelds X1 = cos θ ∂ ∂ + sin θ , ∂x ∂y X2 = and X3 = sin θ ∂ ∂θ ∂ ∂ − cos θ . ∂x ∂y Observe that, similarly to the Heisenberg group, [X1 , X2 ] = X3 .

3. The Haar measure on G coincides with the push-forward of the Lebesgue measure on the Lie algebra g under the exponential map. It is easy to verify that the Jacobian determinant of the dilation δs : G → G is constant, equal to sQ . As with the Heisenberg group, we deﬁne the horizontal gradient of a C 1 function f : G → R by m1 ∇0 f = X1j f X1j . j=1 At various points in this survey we will work in this general setting to emphasize the fact that certain results do not depend on the special structure of H.

Early assumptions – supported by some research – that cortical connectivity runs mostly vertically along the hypercolumns and is severely restricted in horizontal directions (between hypercolumns), were contradicted by later evidence showing “long range horizontal” connectivity in the cortex. This experimental evidence demonstrated the properties of a speciﬁc geometric structure in the ﬁrst layer of the visual cortex associated to intracortical communication. We may mathematically model this structure of the cortex using a sub-Riemannian structure.