52 research outputs found
Closing the Gap Between Short and Long XORs for Model Counting
Many recent algorithms for approximate model counting are based on a
reduction to combinatorial searches over random subsets of the space defined by
parity or XOR constraints. Long parity constraints (involving many variables)
provide strong theoretical guarantees but are computationally difficult. Short
parity constraints are easier to solve but have weaker statistical properties.
It is currently not known how long these parity constraints need to be. We
close the gap by providing matching necessary and sufficient conditions on the
required asymptotic length of the parity constraints. Further, we provide a new
family of lower bounds and the first non-trivial upper bounds on the model
count that are valid for arbitrarily short XORs. We empirically demonstrate the
effectiveness of these bounds on model counting benchmarks and in a
Satisfiability Modulo Theory (SMT) application motivated by the analysis of
contingency tables in statistics.Comment: The 30th Association for the Advancement of Artificial Intelligence
(AAAI-16) Conferenc
Empirical Bounds on Linear Regions of Deep Rectifier Networks
We can compare the expressiveness of neural networks that use rectified
linear units (ReLUs) by the number of linear regions, which reflect the number
of pieces of the piecewise linear functions modeled by such networks. However,
enumerating these regions is prohibitive and the known analytical bounds are
identical for networks with same dimensions. In this work, we approximate the
number of linear regions through empirical bounds based on features of the
trained network and probabilistic inference. Our first contribution is a method
to sample the activation patterns defined by ReLUs using universal hash
functions. This method is based on a Mixed-Integer Linear Programming (MILP)
formulation of the network and an algorithm for probabilistic lower bounds of
MILP solution sets that we call MIPBound, which is considerably faster than
exact counting and reaches values in similar orders of magnitude. Our second
contribution is a tighter activation-based bound for the maximum number of
linear regions, which is particularly stronger in networks with narrow layers.
Combined, these bounds yield a fast proxy for the number of linear regions of a
deep neural network.Comment: AAAI 202
Probabilistic Program Abstractions
Abstraction is a fundamental tool for reasoning about complex systems.
Program abstraction has been utilized to great effect for analyzing
deterministic programs. At the heart of program abstraction is the relationship
between a concrete program, which is difficult to analyze, and an abstract
program, which is more tractable. Program abstractions, however, are typically
not probabilistic. We generalize non-deterministic program abstractions to
probabilistic program abstractions by explicitly quantifying the
non-deterministic choices. Our framework upgrades key definitions and
properties of abstractions to the probabilistic context. We also discuss
preliminary ideas for performing inference on probabilistic abstractions and
general probabilistic programs
Random CNF-XOR Formulas
Boolean Satisfiability (SAT) is fundamental in many diverse areas such as artificial intelligence, formal verification, and biology. Recent universal-hashing based approaches to the problems of sampling and counting crucially depend on the runtime performance of specialized SAT solvers on formulas expressed as the conjunction of both k-CNF constraints and variable-width XOR constraints (known as CNF-XOR formulas), but random CNF-XOR formulas are unexplored in prior work.
In this work, we present the first study of random CNF-XOR formulas. We prove that a phase-transition in the satisfiability of random CNF-XOR formulas exists for k=2 and (when the number of k-CNF constraints is small) for k>2. We empirically demonstrate that a state-of-the-art SAT solver scales exponentially on random CNF-XOR formulas across many clause densities, peaking around the empirical phase-transition location. Finally, we prove that the solution space of random CNF-XOR formulas 'shatters' at all nonzero XOR-clause densities into well-separated components
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