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 Yearling
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2h
answered Homology and (co)Limits
19h
revised On the associative property of a binary operation of the fundamental group.
slight correction to a quotation
19h
revised On the associative property of a binary operation of the fundamental group.
addeda point about aesthetics
1d
answered On the associative property of a binary operation of the fundamental group.
Apr
11
revised fundamental group of the complement of a circle
added emphjasis
Apr
9
comment Fundamental group of R^2-s^1
The questioner seems to mix up $R^2$ and $R^3$. For $R^3$ the question has been answered at math.stackexchange.com/questions/1118477/…
Apr
8
awarded  Yearling
Apr
7
comment The fundamental group of the complement of a circle in 3D-space.
See my answer to math.stackexchange.com/questions/1118477/… .
Apr
4
comment Group Theory via Category Theory
Look also at arXiv:1207.6404
Apr
3
comment Crossed module structure on $\pi_1$-level of any map $f: X\to Y$
Have a look at this paper: Porter, T. {$n$}-types of simplicial groups and crossed {$n$}-cubes. Topology {32}~(1) (1993) 5--24. It may also be somewhere in his "Crossed menagerie" notes.
Apr
2
answered Crossed module structure on $\pi_1$-level of any map $f: X\to Y$
Mar
30
revised Perturbation trick in the proof of Seifert-van-Kampen
some typos and some more use of bold
Mar
28
answered How is the following a CW complex
Mar
28
revised Group Theory via Category Theory
explained a specific construction
Mar
26
revised $2$-Morphisms in the Fundamental $2$-Groupoid
typo to get graphic
Mar
26
answered Group Theory via Category Theory
Mar
26
revised Eckmann-Hilton and higher homotopy groups
added a link
Mar
25
comment The abstract definition of commutative monoids
I can't resist pointing out the article arxiv.org/abs/1405.2889 on "double semigroups" . See also arxiv.org/abs/1501.03690 on double inverse semigroups.
Mar
25
comment Homotopy and Semidirect Product
A minor correction to the answer: the homotopy groups are defined for topological spaces with base point; I think trouble can arise if the distinction is not kept. Pursuing this distinction allows one to think of spaces with many base point, and also spaces with other kinds of structure appropriate to the geometry, such as filtered spaces, and $n$-cubes of spaces, and their appropriate invariants.
Mar
25
answered If $A$, $B$ are path connected and $A \cup B$ is simply connected, $A \cap B$ is path connected