In mathematics, the Parry–Sullivan invariant (or Parry–Sullivan number) is a numerical quantity of interest in the study of incidence matrices in graph theory, and of certain one-dimensional dynamical systems. It provides a partial classification of non-trivial irreducible incidence matrices. It is named after the English mathematician Bill Parry and the American mathematician Dennis Sullivan, who introduced the invariant in a joint paper published in the journal Topology in 1975.
Definition Let A be an n × n incidence matrix. Then the Parry–Sullivan number of A is defined to be
P S ( A ) = det ( I − A ) , {\displaystyle \mathrm {PS} (A)=\det(I-A),}
where I denotes the n × n identity matrix.
Properties It can be shown that, for nontrivial irreducible incidence matrices, flow equivalence is completely determined by the Parry–Sullivan number and the Bowen–Franks group.
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