Arnold Neumaier
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 Aug 17 awarded Popular Question Jun 28 comment Generalization of De Rham cohomology for spinor fields There is an answer at physicsoverflow.org/32072 May 10 comment rigorous treatment of infinitesimal reparametrizations some answers are at physicsoverflow.org/30865 May 7 accepted good books on measures and integration theory in infinite-dimensional spaces Mar 3 awarded Yearling Jan 8 awarded Revival Sep 24 awarded Autobiographer Jul 2 awarded Curious Apr 24 comment Why do Integer Relation Algorithms (e.g. PSLQ) not solve the subset sum problem? @pyramids: 1000 x+ 1001 y = 1 has inifintely many integer solutions but no 0,1 solutions. But even if an equation has a 0,1 solution, why do you expect that an algorithm for finding one of the infinitely many integer solutions should always produce the 01 solution eceot in rare cases?? Apr 24 comment Why do Integer Relation Algorithms (e.g. PSLQ) not solve the subset sum problem? @pyramids: An algorithm for solving the integer constrained problem in polynomial time will just find some integer relation. This is very unlikely to be a (0,1) solution (which might not even exist). It cannot get stuck since it is not based on enumerating near solutions. Apr 20 answered Why do Integer Relation Algorithms (e.g. PSLQ) not solve the subset sum problem? Apr 19 answered Basis reduction and continued fractions Apr 19 answered What is a good introduction to quantities such as the norm of a lattice and of short vectors in the context of lattice reduction? Mar 21 answered What are the properties of this Poisson algebra? Mar 3 awarded Yearling Mar 3 awarded Yearling Dec 2 accepted classical solutions of PDE with mixed boundary conditions Dec 2 comment classical solutions of PDE with mixed boundary conditions Thanks. These and especially their reference to Azzam and Kreyszig are very useful! Nov 13 comment Eigenvalues of a bipartite graph @Shahab: It implies that $k|x|=A|x|$. For if this fails then there is strict inequality in at least one component, and multiplication by $u^T>0$ gives a strict inequality, contradiction. Nov 9 comment Spectrum of a Block Matrix @Timothy: intuitively: Draw a graph of the corresponding quadratic functions and you'll see where the inequalities hold. - formally: complete the squares, take square roots, and discard the signs not compatible with the case.