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Knupfer H.,Hausdorff Center for Mathematics | Muratov C.B.,New Jersey Institute of Technology
Journal of Nonlinear Science | Year: 2011

We investigate the ground state of a uniaxial ferromagnetic plate with perpendicular easy axis and subject to an applied magnetic field normal to the plate. Our interest is in the asymptotic behavior of the energy in macroscopically large samples near the saturation field. We establish the scaling of the critical value of the applied field strength below saturation at which the ground state changes from the uniform to a multidomain magnetization pattern and the leading order scaling behavior of the minimal energy. Furthermore, we derive a reduced sharp interface energy, giving the precise asymptotic behavior of the minimal energy in macroscopically large plates under a physically reasonable assumption of small deviations of the magnetization from the easy axis away from domain walls. On the basis of the reduced energy and by a formal asymptotic analysis near the transition, we derive the precise asymptotic values of the critical field strength at which non-trivial minimizers (either local or global) emerge. The non-trivial minimal energy scaling is achieved by magnetization patterns consisting of long slender needle-like domains of magnetization opposing the applied field. © 2011 Springer Science+Business Media, LLC. Source


Saxena N.,Hausdorff Center for Mathematics | Seshadhri C.,IBM
SIAM Journal on Computing | Year: 2011

We study the problem of polynomial identity testing for depth-3 circuits of degree d and top fanin k. The rank of any such identity is essentially the minimum number of independent variables present. Small bounds on this quantity imply fast deterministic identity testers for these circuits. Dvir and Shpilka [SIAM J. Comput., 36 (2007), pp. 1404-1434] initiated the study of the rank and showed that any depth-3 identity (barring some uninteresting corner cases) has a rank of 2O(k2)(log d)k-2. We show that the rank of a depth-3 identity is at most O(k3 log d). This bound is almost tight, since we also provide an identity of rank ω(k log d). Our rank bound significantly improves (dependence on k exponentially reduced) the best known deterministic black-box identity tests for depth-3 circuits by Karnin and Shpilka [Z. Karnin and A. Shpilka, in Proceedings of the 23rd CCC, 2008, pp. 280-291]. Our techniques also shed light on the factorization pattern of nonzero depth-3 circuits: the rank of linear factors of a simple, minimal, and nonzero depth-3 circuit (over any field) is at most O(k3 log d). The novel feature of this work is a new notion of maps between sets of linear forms, called ideal matchings, used to study depth-3 circuits. We prove interesting structural results about depth-3 identities using these techniques. We believe that these ideas may lead to the goal of a deterministic polynomial time identity test for these circuits. © 2011 Society for Industrial and Applied Mathematics. Source


Saxena N.,Hausdorff Center for Mathematics | Seshadhri C.,Sandia National Laboratories
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2011

Let C be a depth-3 circuit with n variables, degree d and top fanin k (called ΣΠΣ(k,d,n) circuits) over base field FF. It is a major open problem to design a deterministic polynomial time blackbox algorithm that tests if C is identically zero. Klivans & Spielman (STOC 2001) observed that the problem is open even when k is a constant. This case has been subjected to a serious study over the past few years, starting from the work of Dvir & Shpilka (STOC 2005). We give the first polynomial time blackbox algorithm for this problem. Our algorithm runs in time poly(n)dk, regardless of the base field. The only field for which polynomial time algorithms were previously known is FF = QQ (Kayal & Saraf, FOCS 2009, and Saxena & Seshadhri, FOCS 2010). This is the first blackbox algorithm for depth-$3$ circuits that does not use the rank based approaches of Karnin & Shpilka (CCC 2008). We prove an important tool for the study of depth-3 identities. We design a blackbox polynomial time transformation that reduces the number of variables in a ΣΠΣ(k,d,n) circuit to k variables, but preserves the identity structure. © 2011 ACM. Source


Saxena N.,Hausdorff Center for Mathematics | Seshadhri C.,IBM
Proceedings - Annual IEEE Symposium on Foundations of Computer Science, FOCS | Year: 2010

We study the problem of identity testing for depth-3 circuits of top fanin k and degree d. We give a new structure theorem for such identities. A direct application of our theorem improves the known deterministic d kO(k)-time black-box identity test over rationals (Kayal & Saraf, FOCS 2009) to one that takes dO(k2)-time. Our structure theorem essentially says that the number of independent variables in a real depth-3 identity is very small. This theorem affirmatively settles the strong rank conjecture posed by Dvir & Shpilka (STOC 2005). We devise a powerful algebraic framework and develop tools to study depth-3 identities. We use these tools to show that any depth-3 identity contains a much smaller nucleus identity that contains most of the "complexity" of the main identity. The special properties of this nucleus allow us to get almost optimal rank bounds for depth-3 identities. © 2010 IEEE. Source


de Philippis G.,Hausdorff Center for Mathematics | Figalli A.,University of Texas at Austin
Archive for Rational Mechanics and Analysis | Year: 2014

We prove higher integrability for the gradient of local minimizers of the Mumford-Shah energy functional, providing a positive answer to a conjecture of De Giorgi (Free discontinuity problems in calculus of variations. Frontiers in pure and applied mathematics, North-Holland, Amsterdam, pp 55-62, 1991). © 2014 Springer-Verlag Berlin Heidelberg. Source

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