]. S. Ada90 and . Adams, Trees and amenable equivalence relations. Ergodic Theory Dynamical Systems, pp.1-14, 1990.

]. A. Alv08a and . Alvarez, Théorème de Kurosh pour les relations d'´ equivalence boréliennes, 2008.

]. A. Alv08b and . Alvarez, Une théorie de Bass-Serre pour les relations d'´ equivalence et les groupo¨?desgroupo¨?des boréliens, 2008.

R. [. Adams and . Spatzier, Kazhdan Groups, Cocycles and Trees, American Journal of Mathematics, vol.112, issue.2, pp.271-287, 1990.
DOI : 10.2307/2374716

M. E. Bekka and A. Valette, Group cohomology, harmonic functions and the first L 2 -Betti number, Potential Analysis, vol.6, issue.4, pp.313-326, 1997.
DOI : 10.1023/A:1017974406074

J. Cheeger and M. Gromov, L2-Cohomology and group cohomology, Topology, vol.25, issue.2, pp.189-215, 1986.
DOI : 10.1016/0040-9383(86)90039-X

I. Chifan and C. Houdayer, Prime factors and amalgamated free products, 2008.

D. Gaboriau, Mercuriale de groupes et de relations, Comptes Rendus de l'Acad??mie des Sciences - Series I - Mathematics, vol.326, issue.2, pp.219-222, 1998.
DOI : 10.1016/S0764-4442(97)89474-8

]. D. Gab00 and . Gaboriau, Coût des relations d'´ equivalence et des groupes, Invent. Math, vol.139, issue.1, pp.41-98, 2000.

]. D. Gab02a and . Gaboriau, Invariants L 2 de relations d'´ equivalence et de groupes, Publ. Math. Inst. HautesÉtudesHautes´HautesÉtudes Sci, vol.95, pp.93-150, 2002.

]. D. Gab02b and . Gaboriau, On orbit equivalence of measure preserving actions, Rigidity in dynamics and geometry, pp.167-186

]. D. Gab05 and . Gaboriau, Examples of groups that are measure equivalent to the free group. Ergodic Theory Dynam, Systems, vol.25, issue.6, pp.1809-1827, 2005.

D. Gaboriau, Relative property (T) actions and trivial outer automorphism groups. preprint, 2008.
DOI : 10.1016/j.jfa.2010.09.013
URL : https://hal.archives-ouvertes.fr/ensl-00269298

G. Hjorth, A lemma for cost attained, Annals of Pure and Applied Logic, vol.143, issue.1-3, pp.87-102, 2006.
DOI : 10.1016/j.apal.2005.05.034

J. [. Ioana, S. Peterson, and . Popa, Amalgamated free products of weakly rigid factors and calculation of their symmetry groups, Acta Mathematica, vol.200, issue.1, 2005.
DOI : 10.1007/s11511-008-0024-5

A. [. Jackson, A. Kechris, and . Louveau, COUNTABLE BOREL EQUIVALENCE RELATIONS, Journal of Mathematical Logic, vol.02, issue.01, pp.1-80, 2002.
DOI : 10.1142/S0219061302000138

A. Kurosch, Die Untergruppen der freien Produkte von beliebigen Gruppen, Mathematische Annalen, vol.5, issue.1, pp.647-660, 1934.
DOI : 10.1007/BF01449159

G. Levitt, On the cost of generating an equivalence relation. Ergodic Theory Dynam, Systems, vol.15, issue.6, pp.1173-1181, 1995.

W. Lück, L 2 -invariants: theory and applications to geometry and Ktheory, 2002.

N. Monod and Y. Shalom, Orbit equivalence rigidity and bounded cohomology, Annals of Mathematics, vol.164, issue.3, pp.825-878, 2006.
DOI : 10.4007/annals.2006.164.825
URL : http://arxiv.org/abs/math/0703165

D. Ornstein and B. Weiss, Ergodic theory of amenable group actions. I: The Rohlin lemma, Bulletin of the American Mathematical Society, vol.2, issue.1, pp.161-164, 1980.
DOI : 10.1090/S0273-0979-1980-14702-3

]. S. Pop06 and . Popa, On a class of type II 1 factors with Betti numbers invariants, Ann. of Math, vol.163, issue.23, pp.809-899, 2006.

]. Ser77 and . Serre, Arbres, amalgames, SL 2, Astérisque. S.M.F, vol.46, 1977.

J. [. Slaman and . Steel, Definable functions on degrees, Lecture Notes in Math, vol.1333, pp.37-55
DOI : 10.1017/CBO9781139519694.015

B. Weiss, Measurable dynamics, Conference in modern analysis and probability, pp.395-421, 1982.
DOI : 10.1090/conm/026/737417