Reputation
21,185
Next tag badge:
99/100 score
24/20 answers
Badges
9 52 147
Impact
~561k people reached

Aug
9
comment Question about the sum of chain groups
Actually, I do draw pictures. I've been following your advice on more than one occasion... and I appreciate your help! I think I should chuck Hatcher, every now and again there is a passage in it that is utterly unhelpful : (
Aug
9
accepted Question about the sum of chain groups
Aug
9
comment Question about the sum of chain groups
Why did you down vote?
Aug
9
comment Question about the sum of chain groups
It's simplices I think.
Aug
9
comment Question about the sum of chain groups
Actually, I think elements in $C(A) \oplus C(B)$ look like $(a,b)$ and the corresponding element in $C(A+B)$ looks like $a+b$... but they represent the same element
Aug
9
comment Question about the sum of chain groups
I have taken this notation from Hatcher p. 149, I think $A + B$ means $a + b$ for chains $a \in A$ and $b \in B$ where $a = \sum_i n_i \sigma_i$ and $b = \sum_i m_i \tau_i$
Aug
9
asked Question about the sum of chain groups
Aug
8
comment Homology of disjoint union is direct sum of homologies
@wckronholm: thanks for the hint!
Aug
8
comment Homology of disjoint union is direct sum of homologies
@gary, hey thanks for the hint!
Aug
8
comment Homology of disjoint union is direct sum of homologies
@Theo, ok, I'll do that.
Aug
8
asked Homology of disjoint union is direct sum of homologies
Aug
8
answered Question about deformation retracts and neighbourhoods
Aug
8
comment Question about deformation retracts and neighbourhoods
Yes, Prop. 2.22 it is. But now it's clear, I had written down the definition of good and omitted that it had to be closed. If $V$ is a neighbourhood of $A$ and $A$ is closed then $\bar{A} \subset A \subset int(V)$.
Aug
8
comment Question about deformation retracts and neighbourhoods
Oh. It has to be closed too, not just a deformation retract! You're right. Thank you!
Aug
8
comment Question about deformation retracts and neighbourhoods
When you write ball you mean disc, i.e. $D^n$, not $S^n$, right? Then the closure of the point is not in the interior so that is a counter example. Now I'm confused because on p 124 Hatcher applies the excision theorem but $\bar{A} \subset int(V)$ doesn't necessarily hold : (
Aug
8
comment Question about deformation retracts and neighbourhoods
No, $V$ might not be $X$.
Aug
8
revised Question about deformation retracts and neighbourhoods
added 64 characters in body
Aug
8
asked Question about deformation retracts and neighbourhoods
Aug
4
comment Boundary of a simplex
What do you mean by the parametrizations of $f_{i,q}$? I know nothing about manifolds yet unfortunately.
Aug
4
accepted Reduced homology