Let F2 be the free group generated by x and y. In this article, we prove that the commutator of xm and yn is a product of two squares if and only if mn is even. We also show using topological methods that there are infinitely many obstructions for an element in F2 to be a product of two squares.
Sarkar, Sucharit  1
@article{10_2140_agt_2004_4_595,
author = {Sarkar, Sucharit},
title = {Commutators and squares in free groups},
journal = {Algebraic and Geometric Topology},
pages = {595--602},
year = {2004},
volume = {4},
number = {1},
doi = {10.2140/agt.2004.4.595},
url = {http://geodesic.mathdoc.fr/articles/10.2140/agt.2004.4.595/}
}
Sarkar, Sucharit. Commutators and squares in free groups. Algebraic and Geometric Topology, Tome 4 (2004) no. 1, pp. 595-602. doi: 10.2140/agt.2004.4.595
[1] , Powers of commutators as products of squares, Int. J. Math. Math. Sci. 31 (2002) 635
[2] , , Commutators as products of squares, Proc. Amer. Math. Soc. 39 (1973) 267
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