An simple intermediate step of the proof that $partial^2 = 0$ in the case of singular homology
$begingroup$
Let $Delta_q$ be the standard $q$-simplex. The function $F_q^s : Delta_{q-1} to Delta_q$ is the $s$-th face of $Delta_q$, defined as the correstriction to $Delta_q$ of $(e_0,dots,e_{s-1},hat{e_s},e_{s+1},dots,e_q)$, the map $Delta_{q-1} to mathbb{R}^q$ that sends $e_i$ to itself for $i < s$ and $e_i$ to $e_{i+1}$ if $i geq s$.
I'm stuck on trying to prove the following elementary result,
Lemma. Let $0 leq j < i leq q$. Then $F_q^{i}F_{q-1}^{j} = F_q^{j}F_{q-1}^{i-1}$.
I apologize for the simplicity of the question, but I cannot seem to pinpoint where am I making the mistake, so any help is greatly appreciated.
By a direct calculation,
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq j, l geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq i$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l < j$}\
e_{l+2} text{ if $l geq i-1, l geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
These don't seem to coincide. Where have I gone wrong?
algebraic-topology proof-explanation
$endgroup$
add a comment |
$begingroup$
Let $Delta_q$ be the standard $q$-simplex. The function $F_q^s : Delta_{q-1} to Delta_q$ is the $s$-th face of $Delta_q$, defined as the correstriction to $Delta_q$ of $(e_0,dots,e_{s-1},hat{e_s},e_{s+1},dots,e_q)$, the map $Delta_{q-1} to mathbb{R}^q$ that sends $e_i$ to itself for $i < s$ and $e_i$ to $e_{i+1}$ if $i geq s$.
I'm stuck on trying to prove the following elementary result,
Lemma. Let $0 leq j < i leq q$. Then $F_q^{i}F_{q-1}^{j} = F_q^{j}F_{q-1}^{i-1}$.
I apologize for the simplicity of the question, but I cannot seem to pinpoint where am I making the mistake, so any help is greatly appreciated.
By a direct calculation,
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq j, l geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq i$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l < j$}\
e_{l+2} text{ if $l geq i-1, l geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
These don't seem to coincide. Where have I gone wrong?
algebraic-topology proof-explanation
$endgroup$
1
$begingroup$
Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
$endgroup$
– F M
Dec 17 '18 at 15:58
add a comment |
$begingroup$
Let $Delta_q$ be the standard $q$-simplex. The function $F_q^s : Delta_{q-1} to Delta_q$ is the $s$-th face of $Delta_q$, defined as the correstriction to $Delta_q$ of $(e_0,dots,e_{s-1},hat{e_s},e_{s+1},dots,e_q)$, the map $Delta_{q-1} to mathbb{R}^q$ that sends $e_i$ to itself for $i < s$ and $e_i$ to $e_{i+1}$ if $i geq s$.
I'm stuck on trying to prove the following elementary result,
Lemma. Let $0 leq j < i leq q$. Then $F_q^{i}F_{q-1}^{j} = F_q^{j}F_{q-1}^{i-1}$.
I apologize for the simplicity of the question, but I cannot seem to pinpoint where am I making the mistake, so any help is greatly appreciated.
By a direct calculation,
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq j, l geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq i$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l < j$}\
e_{l+2} text{ if $l geq i-1, l geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
These don't seem to coincide. Where have I gone wrong?
algebraic-topology proof-explanation
$endgroup$
Let $Delta_q$ be the standard $q$-simplex. The function $F_q^s : Delta_{q-1} to Delta_q$ is the $s$-th face of $Delta_q$, defined as the correstriction to $Delta_q$ of $(e_0,dots,e_{s-1},hat{e_s},e_{s+1},dots,e_q)$, the map $Delta_{q-1} to mathbb{R}^q$ that sends $e_i$ to itself for $i < s$ and $e_i$ to $e_{i+1}$ if $i geq s$.
I'm stuck on trying to prove the following elementary result,
Lemma. Let $0 leq j < i leq q$. Then $F_q^{i}F_{q-1}^{j} = F_q^{j}F_{q-1}^{i-1}$.
I apologize for the simplicity of the question, but I cannot seem to pinpoint where am I making the mistake, so any help is greatly appreciated.
By a direct calculation,
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq j, l geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i$}\
e_{l+2} text{ if $l geq i$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l < j$}\
e_{l+2} text{ if $l geq i-1, l geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
These don't seem to coincide. Where have I gone wrong?
algebraic-topology proof-explanation
algebraic-topology proof-explanation
edited Dec 16 '18 at 6:39
Guido A.
asked Dec 16 '18 at 6:33
Guido A.Guido A.
7,8701730
7,8701730
1
$begingroup$
Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
$endgroup$
– F M
Dec 17 '18 at 15:58
add a comment |
1
$begingroup$
Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
$endgroup$
– F M
Dec 17 '18 at 15:58
1
1
$begingroup$
Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
$endgroup$
– F M
Dec 17 '18 at 15:58
$begingroup$
Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
$endgroup$
– F M
Dec 17 '18 at 15:58
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
You did not correctly calculate $F^s_q(e_{l+1})$. You have
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l+1 < i$}\
e_{l+2} text{ if $l geq j, l+1 geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i-1$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l+1< j$}\
e_{l+2} text{ if $l geq i-1, l+1 geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
$endgroup$
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
add a comment |
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1 Answer
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$begingroup$
You did not correctly calculate $F^s_q(e_{l+1})$. You have
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l+1 < i$}\
e_{l+2} text{ if $l geq j, l+1 geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i-1$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l+1< j$}\
e_{l+2} text{ if $l geq i-1, l+1 geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
$endgroup$
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
add a comment |
$begingroup$
You did not correctly calculate $F^s_q(e_{l+1})$. You have
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l+1 < i$}\
e_{l+2} text{ if $l geq j, l+1 geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i-1$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l+1< j$}\
e_{l+2} text{ if $l geq i-1, l+1 geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
$endgroup$
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
add a comment |
$begingroup$
You did not correctly calculate $F^s_q(e_{l+1})$. You have
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l+1 < i$}\
e_{l+2} text{ if $l geq j, l+1 geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i-1$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l+1< j$}\
e_{l+2} text{ if $l geq i-1, l+1 geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
$endgroup$
You did not correctly calculate $F^s_q(e_{l+1})$. You have
$$
begin{align}
F_q^{i}F_{q-1}^{j}(e_l) &= cases{F_q^{i}(e_l) text{ if $l < j$}\ F_q^{i}(e_{l+1}) text{ if $l geq j$}} \&=
cases{
e_l quad text{ if $l < j, l< i$}\
e_{l+1} text{ if $l < j, l geq i$}\
e_{l+1} text{ if $l geq j, l+1 < i$}\
e_{l+2} text{ if $l geq j, l+1 geq i$}} \&=
cases{
e_l quad text{ if $l < j$}\
e_{l+1} text{ if $l geq j, l < i-1$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
and
$$
begin{align}
F_q^{j}F_{q-1}^{i-1}(e_l) &= cases{F_q^{j}(e_l) text{ if $l < i-1$}\ F_q^{j}(e_{l+1}) text{ if $l geq i-1$}} \ &=
cases{
e_l quad text{ if $l < i-1, l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+1} text{ if $l geq i-1, l+1< j$}\
e_{l+2} text{ if $l geq i-1, l+1 geq j$}} \ &=
cases{
e_l quad text{ if $l< j$}\
e_{l+1} text{ if $l < i-1, l geq j$}\
e_{l+2} text{ if $l geq i-1$}}
end{align}
$$
edited Dec 16 '18 at 9:45
answered Dec 16 '18 at 9:38
Paul FrostPaul Frost
11.5k3934
11.5k3934
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
add a comment |
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
$begingroup$
I see it now! I had rewritten this many times, but it seems that I had fixed my mistake already. Thanks a lot!
$endgroup$
– Guido A.
Dec 17 '18 at 1:55
add a comment |
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1
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Many homology theories are described by a collection of maps called face and degeneracy maps, satisfying a set of axioms called simplicial identities (you may read more about them at the nLab). The ones you posted are precisely the ones which guarantee that their alternating sum is a differential.
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– F M
Dec 17 '18 at 15:58