Equality of fractions in localization as quotient
$begingroup$
Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.
Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.
So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.
What is my mistake?
commutative-algebra localization
$endgroup$
add a comment |
$begingroup$
Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.
Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.
So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.
What is my mistake?
commutative-algebra localization
$endgroup$
add a comment |
$begingroup$
Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.
Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.
So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.
What is my mistake?
commutative-algebra localization
$endgroup$
Let $Ssubset A$ be a subset of a commutative ring. Consider the polynomial algebra $A[x_smid sin S]$ and define the localization $A_S$ as the quotient by the ideal $ (sx_s-1mid sin S)vartriangleleft A[x_smid sin S]$.
Let us write $frac as$ for the equivalence class of $ax_s$ in the quotient $A_S$. I would like to prove that $frac as=frac bt$ iff $exists uin S$ such that $u(at-bs)=0$, like in the construction of localization as a quotient of $Atimes S$.
So we have $frac as-frac bt=0in A_S$ which is equivalent to $ax_s-bx_tin I$. By degree considerations I think we must have $ax_s-bx_t=c(sx_s-1)-c(tx_t-1)$ for some $cin A$, and by comparing coefficients we have $a=cs,b=ct$. Multiplying by $t,s$ respectively we find $at=cst=cts=bs$, whence $at-bs=0$ without any $uin S$.
What is my mistake?
commutative-algebra localization
commutative-algebra localization
asked Feb 1 at 0:38


ArrowArrow
5,20831546
5,20831546
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$begingroup$
I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.
Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.
Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.
Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.
I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:
Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.
To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$
You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.
$endgroup$
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$begingroup$
I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.
Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.
Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.
Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.
I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:
Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.
To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$
You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.
$endgroup$
add a comment |
$begingroup$
I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.
Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.
Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.
Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.
I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:
Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.
To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$
You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.
$endgroup$
add a comment |
$begingroup$
I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.
Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.
Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.
Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.
I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:
Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.
To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$
You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.
$endgroup$
I think the main mistake is in the statement "by degree considerations..." I did not verify that it is true for integral domains, but these considerations are more complicated in the presence of zerodivisors.
Let's consider a (classical) localization in a ring $A$ with zerodivisors. For example, let $A = mathbb{Z}[y]/(2y)$ and invert $2$. Then in the localization $y=0$.
Now consider your polynomial-ring construction, in which $2x_2 - 1 = 0$ and (multiplying by $y$) $$2x_2 y - y = 0,$$ i.e. $y=0$. This looks like a separate, more minor, problem: it is a relation not of the form $$frac{a}{s}=frac{b}{t}$$ for $s,t in S$ (you might want to take $1 in S$.) I'll assume we took $1in S$, and use a corresponding variable $x_1$ below.
Anyway, $y=0$ and in particular we have for example $$yx_2 - 0x_1 = yx_2 in I.$$ So by your degree considerations, we should get $yx_2 = c(2x_2 - 1) - c(1x_1-1)$ in $A[x_1, x_2]$ for some $cin A$. This is not the case. To exhibit this relation as an element of $I$ you need something like $yx_2 cdot (2x_2 - 1),$ where $yx_2$ is not of degree $0$, hence not in $A$, so the degree considerations fail.
I previously wrote a "correction" to your proof, but it was mainly nonsense. There was some germ of a good idea hidden there, though. Here is a sketch of a correct proof:
Consider a ring $A$ and a module $M$ over it, and take $$(Moplus M) / ((x,sx))$$ for some fixed $sin A$. What is the submodule generated by the first coordinate? If an element $x$ is annihilated by $s$, then $$(x,0) = (x,0) - (x,sx) = (0,0)$$ in the quotient. It can be verified directly that the submodule is just $$frac{M}{text{ker}(s:Mrightarrow M)}.$$ What if we take $$bigoplus_{i,j ge 0} M$$ and mod out by elements of the form $$se_{i+1,j}-e_{i,j},quad te_{i,j+1}-e_{i,j}?$$ I claim the $e_{0,0}$ coordinate then generates a module isomorphic to $$frac{M}{bigcup_n text{ker}(st)^n:Mrightarrow M},$$ et cetera.
To see this, consider the colimit explicitly. If an element of the submodule generated by $M_{0,0}$ in the quotient vanishes, it vanishes already in some f.g. submodule, which is contained in some direct sum of a finite set of $M_{i,j}$, and hence in a "rectangular" submodule $$bigoplus_{0le i,jle n} M_{i,j}.$$ Any element here can be "pushed" to one supported only in $M_{n,n}$, and it is easy to check what it means that it vanishes there. The same argument generalizes to higher "dimensions" $$bigoplus M_{i,j,k}.$$
You want to take $$B:= Aleft[{x_s}_{sin S}right]$$ and consider some ideal generated by elements of the form $(sx_s - 1)$. Considering the polynomial ring as a free $A$-module, the required statement follows from the previous one, once one shows the ideal is generated as an $A$-submodule of $B$ by elements of the same form as above. This is not so bad since $$(sx_s -1)(tx_t - 1) = (stx_sx_t - 1) - (sx_s -1) - (tx_t - 1)$$ is already a sum of three relations, and the same will occur for more complicated monomials in place of $x_s$ and $x_t$. Multiplying a relation by a monomial also gives you a sum of relations in the quotient submodule of $$bigoplus_{i,j}M_i,j$$ above.
edited Feb 3 at 13:49
answered Feb 1 at 22:40
gyashfegyashfe
963
963
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