Abstract
The set \(\mathcal {D}_n\) of all difunctional relations on an n element set is an inverse semigroup under a variation of the usual composition operation. We solve an open problem of Kudryavtseva and Maltcev (Publ Math Debrecen 78(2):253–282, 2011), which asks: What is the rank (smallest size of a generating set) of \(\mathcal {D}_n\)? Specifically, we show that the rank of \(\mathcal {D}_n\) is \(B(n)+n\), where B(n) is the nth Bell number. We also give the rank of an arbitrary ideal of \(\mathcal {D}_n\). Although \(\mathcal {D}_n\) bears many similarities with families such as the full transformation semigroups and symmetric inverse semigroups (all contain the symmetric group and have a chain of \(\mathscr {J}\)-classes), we note that the fast growth of \({\text {rank}}(\mathcal {D}_n)\) as a function of n is a property not shared with these other families.
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Notes
We note that Proposition 7 in an earlier version of [11], available at http://arxiv.org/pdf/math/0602623v1.pdf, leads to a lower bound for \({\text {rank}}(\mathcal {D}_n)\) that is fairly close to the precise value.
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Communicated by Norman R. Reilly.
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East, J., Vernitski, A. Ranks of ideals in inverse semigroups of difunctional binary relations. Semigroup Forum 96, 21–30 (2018). https://doi.org/10.1007/s00233-017-9846-9
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DOI: https://doi.org/10.1007/s00233-017-9846-9