78 research outputs found
From Sets to Multisets: Provable Variational Inference for Probabilistic Integer Submodular Models
Submodular functions have been studied extensively in machine learning and
data mining. In particular, the optimization of submodular functions over the
integer lattice (integer submodular functions) has recently attracted much
interest, because this domain relates naturally to many practical problem
settings, such as multilabel graph cut, budget allocation and revenue
maximization with discrete assignments. In contrast, the use of these functions
for probabilistic modeling has received surprisingly little attention so far.
In this work, we firstly propose the Generalized Multilinear Extension, a
continuous DR-submodular extension for integer submodular functions. We study
central properties of this extension and formulate a new probabilistic model
which is defined through integer submodular functions. Then, we introduce a
block-coordinate ascent algorithm to perform approximate inference for those
class of models. Finally, we demonstrate its effectiveness and viability on
several real-world social connection graph datasets with integer submodular
objectives
Submodular Minimization Under Congruency Constraints
Submodular function minimization (SFM) is a fundamental and efficiently
solvable problem class in combinatorial optimization with a multitude of
applications in various fields. Surprisingly, there is only very little known
about constraint types under which SFM remains efficiently solvable. The
arguably most relevant non-trivial constraint class for which polynomial SFM
algorithms are known are parity constraints, i.e., optimizing only over sets of
odd (or even) cardinality. Parity constraints capture classical combinatorial
optimization problems like the odd-cut problem, and they are a key tool in a
recent technique to efficiently solve integer programs with a constraint matrix
whose subdeterminants are bounded by two in absolute value.
We show that efficient SFM is possible even for a significantly larger class
than parity constraints, by introducing a new approach that combines techniques
from Combinatorial Optimization, Combinatorics, and Number Theory. In
particular, we can show that efficient SFM is possible over all sets (of any
given lattice) of cardinality r mod m, as long as m is a constant prime power.
This covers generalizations of the odd-cut problem with open complexity status,
and with relevance in the context of integer programming with higher
subdeterminants. To obtain our results, we establish a connection between the
correctness of a natural algorithm, and the inexistence of set systems with
specific combinatorial properties. We introduce a general technique to disprove
the existence of such set systems, which allows for obtaining extensions of our
results beyond the above-mentioned setting. These extensions settle two open
questions raised by Geelen and Kapadia [Combinatorica, 2017] in the context of
computing the girth and cogirth of certain types of binary matroids
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