P. Bennett, Amalgamated Free Products of Inverse Semigroups, Journal of Algebra, vol.198, issue.2, pp.499-534, 1994.
DOI : 10.1006/jabr.1997.7155

J. Birget, S. Margolis, and J. Meakin, ON THE WORD PROBLEM FOR TENSOR PRODUCTS AND AMALGAMS OF MONOIDS, International Journal of Algebra and Computation, vol.09, issue.03n04, pp.271-294, 1999.
DOI : 10.1142/S0218196799000187

A. Cherubini, J. Meakin, and B. Piochi, Amalgams of free inverse semigroups, Semigroup Forum, vol.63, issue.6, pp.199-220, 1997.
DOI : 10.1007/BF02676602

T. E. Hall, Free products with amalgamation of inverse semigroups, Journal of Algebra, vol.34, issue.3, pp.375-385, 1975.
DOI : 10.1016/0021-8693(75)90164-7

J. M. Howie, Fundamentals of semigroup theory Amalgamations: a survey, in Semigroups: algebraic theory and applications to formal languages and codes, ) World Scientific, pp.125-132, 1994.

P. R. Jones, S. W. Margolis, J. C. Meakin, and J. B. Stephen, Free products of inverse semigroups II, Glasgow Math, J, vol.33, pp.373-387, 1991.

S. W. Margolis and J. C. Meakin, Inverse monoids trees and context-free languages, Trans. Amer. Math. Soc, vol.335, issue.1, pp.259-276, 1993.

W. D. Munn, Free inverse semigroups, Proc. London Math. Soc, vol.30, pp.385-404, 1974.

N. Petrich, Inverse semigroups, Semigroup Forum, vol.32, issue.1, 1984.
DOI : 10.1007/BF02575538

J. B. Stephen, Presentations of inverse monoids, Journal of Pure and Applied Algebra, vol.63, issue.1, pp.81-112, 1990.
DOI : 10.1016/0022-4049(90)90057-O

. Pisa, Italy E-mail address: mazzucchelli@mail.dm.unipi, Dipartimento di Matematica " F. Brioschi " Politecnico di Milano Via Bonardi 9