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arXiv 2005.11874math.DG
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黎曼流形的一个Dehn型量

A Dehn type quantity for Riemannian manifolds

Oliver Knill

摘要:

我们研究了紧致2d黎曼流形M的泛函Y(M)= int_MK(x)dV(x),其中K(x)=(2d)!(d!)^- 1(4pi)^-dint_Tprod_k= 1^dK_t_2k,t_2k +1(x)dt涉及所有正交标架t=(t_1,.,t_2d)。离散形式Y_disc(M),其中K_d(x)=(d!)1(4pi)^-d sum_sigma prod_k=1^d K_sigma(2k-1),sigma(2k)对所有排列sigma求和1,.,2d.与Gauss-Bonnet-Chern提出的欧拉示性数int_M K_GBC dV=X(M)不同,Y或Y_disc与一般的度规有关。我们对Y(M)-X(M)感兴趣,因为如果M具有曲率符号e,则Y(M)e^d和Y_disc(M)是正的,而X(M)e^d>0仅是负的。我们在一些具体的例子中计算Y_disc,例如2d-球面、4-流形CP ^2、6-流形SO(4)或8-流形SU(3)。

英文摘要:

We look at the functional Y(M) = int_M K(x) dV(x) for compact Riemannian 2d-manifolds M, where K(x) = (2d)! (d!)^-1 (4pi)^-d int_T prod_k=1^d K_t_2k,t_2k+1(x) dt involves products of d sectional curvatures K_ij(x) averaged over the space T sim O(2d) of all orthonormal frames t=(t_1, ... ,t_2d). A discrete version Y_disc(M) with K_d(x) = (d!)^-1 (4pi)^-d sum_sigma prod_k=1^d K_sigma(2k-1),sigma(2k) sums over all permutations sigma of 1,..,2d. Unlike Euler characteristic which by Gauss-Bonnet-Chern is int_M K_GBC dV=X(M), the quantities Y or Y_disc are in general metric dependent. We are interested in Y(M)-X(M) because if M has curvature sign e, then Y(M) e^d and Y_disc(M) are positive while X(M) e^d>0 is only conjectured. We compute Y_disc in a few concrete examples like 2d-spheres, the 4-manifold CP^2, the 6 manifold SO(4) or the 8-manifold SU(3).

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