The (tree) amplituhedron \mathcal{A}_{n,k,m} is the image in the Grassmannian \mathrm {Gr}_{k,k+m} of the totally nonnegative Grassmannian \mathrm {Gr}_{k,n}^{\geq 0} , under a (map induced by a) linear map which is totally positive. It was introduced by Arkani-Hamed and Trnka in 2013 in order to give a geometric basis for the computation of scattering amplitudes in planar \mathcal{N}=4 supersymmetric Yang–Mills theory. In the case relevant to physics ( m=4 ), there is a collection of recursively-defined 4k -dimensional BCFW cells in \mathrm {Gr}_{k,n}^{\geq 0} , whose images conjecturally "triangulate" the amplituhedron – that is, their images are disjoint and cover a dense subset of \mathcal{A}_{n,k,4} . In this paper, we approach this problem by first giving an explicit (as opposed to recursive) description of the BCFW cells. We then develop sign-variational tools which we use to prove that when k=2 , the images of these cells are disjoint in \mathcal{A}_{n,k,4} . We also conjecture that for arbitrary even m , there is a decomposition of the amplituhedron \mathcal{A}_{n,k,m} involving precisely M\big(k, n-k-m, \frac{m}{2}\big) top-dimensional cells (of dimension km ), where M(a,b,c) is the number of plane partitions contained in an a \times b \times c box. This agrees with the fact that when m=4 , the number of BCFW cells is the Narayana number N_{n-3,k+1} = \frac{1}{n-3}\binom{n-3}{k+1}\binom{n-3}{k} .
In their foundational work, List and Pettit formalized the judgment aggregation framework and showed that the preference aggregation framework from social choice theory can be mapped into it, arguing that the reverse was not possible. We show that a natural extension of a graph-theoretic representation of the preference aggregation framework indeed allows us to embed also the judgment aggregation framework. Moreover we show that many concepts from the two original frameworks match up under the new one, show that it is possible to detect "logical consistency" with graph-theoretical properties, and give a new nuanced comparison between the doctrinal paradox and the Condorcet paradox.
It is a common belief that Byzantine fault-tolerant solutions for consensus are significantly slower than their crash fault-tolerant counterparts. Indeed, in PBFT, the most widely known Byzantine fault-tolerant consensus protocol, it takes three message delays to decide a value, in contrast with just two in Paxos. This motivates the search for fast Byzantine consensus algorithms that can produce decisions after just two message delays in the common case, e.g., under the assumption that the current leader is correct and not suspected by correct processes. The (optimal) two-step latency comes with the cost of lower resilience: fast Byzantine consensus requires more processes to tolerate the same number of faults. In particular, 5 + 1 processes were claimed to be necessary to tolerate Byzantine failures.In this paper, we present a fast Byzantine consensus algorithm that relies on just 5 − 1 processes. Moreover, we show that 5 − 1 is the tight lower bound, correcting a mistake in the earlier work. While the difference of just 2 processes may appear insignificant for large values of , it can be crucial for systems of a smaller scale. In particular, for = 1, our algorithm requires only 4 processes, which is optimal for any (not necessarily fast) partially synchronous Byzantine consensus algorithm.
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Adinkras are combinatorial objects developed to study supersymmetry representations. Gates et al. introduced the gadget as a function of pairs of adinkras, obtaining some mysterious results for (n = 4, k = 1) adinkras with computer-aided computation. Specifically, very few values of the gadget actually appear, suggesting a great deal of symmetry in these objects. In this paper, we compute gadgets symbolically and explain some of these observed phenomena with group theory and combinatorics. Guided by this work, we give some suggestions for generalizations of the gadget to other values of the n and k parameters.
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