[1] Hatcher, A. 2001. Algebraic Topology (textbook on line)., Cambridge University
Press; Cambridge, UK., 405 pages.
[2] Ronald Brown: Topology and Groupoids, BookSurge LLC (2006).
[3] Ronald Brown R, P.J. Higgins, and R. Sivera.: “Non-Abelian algebraic topology”,
(2008).
[4] R. Brown and J.-L. Loday: Homotopical excision, and Hurewicz theorems, for
n-cubes of spaces, Proc. London Math. Soc., 54:(3), 176-192, (1987).
[5] R. Brown and J.-L. Loday: Van Kampen Theorems for diagrams of spaces, Topology,
26: 311-337 (1987).
[6] R. Brown and C.B. Spencer: Double groupoids and crossed modules, Cahiers Top.
Géom. Diff., 17 (1976), 343-362.
[7] Allain Connes: Noncommutative Geometry, Academic Press 1994.
[8] May, J.P. 1999, A Concise Course in Algebraic Topology., The University of Chicago
Press: Chicago