Thomas, Simon; Schneider, Scott (2012), "Countable Borel equivalence relations", in Cummings, James; Schimmerling, Ernest (eds.), Appalachian Set Theory: 2006-2012, London Mathematical Society Lecture Note Series, vol. 406, Cambridge University Press, pp. 25–62, CiteSeerX10.1.1.648.3113, doi:10.1017/CBO9781139208574.003, ISBN9781107608504. On p. 46, Thomas and Schneider refer to "...this failure to classify even the rank 2 groups in a satisfactory way..."
doi.org
Thomas, Simon; Schneider, Scott (2012), "Countable Borel equivalence relations", in Cummings, James; Schimmerling, Ernest (eds.), Appalachian Set Theory: 2006-2012, London Mathematical Society Lecture Note Series, vol. 406, Cambridge University Press, pp. 25–62, CiteSeerX10.1.1.648.3113, doi:10.1017/CBO9781139208574.003, ISBN9781107608504. On p. 46, Thomas and Schneider refer to "...this failure to classify even the rank 2 groups in a satisfactory way..."
psu.edu
citeseerx.ist.psu.edu
Thomas, Simon; Schneider, Scott (2012), "Countable Borel equivalence relations", in Cummings, James; Schimmerling, Ernest (eds.), Appalachian Set Theory: 2006-2012, London Mathematical Society Lecture Note Series, vol. 406, Cambridge University Press, pp. 25–62, CiteSeerX10.1.1.648.3113, doi:10.1017/CBO9781139208574.003, ISBN9781107608504. On p. 46, Thomas and Schneider refer to "...this failure to classify even the rank 2 groups in a satisfactory way..."