Well-Ordering Principle implies Zorn's Lemma
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Well-Ordering Principle implies Zorn's Lemma
Does my attempt look fine or contain logical flaws/gaps? Any suggestion is greatly appreciated. Thank you for your help!
My attempt:
Let $(A,preccurlyeq)$ be a partially ordered set in which every chain has an upper bound.
By Well-Ordering Principle, there is a well-ordering $preccurlyeq'$ on $A$. Let $V$ be the class of all sets and $rm Ord$ be the class of all ordinals. First, we define function $f:mathcal{P}(A)setminus{emptyset} to A$ by $f(X)=min X$ (with regard to $preccurlyeq'$).
Next, we define function $G:V to V$ by $G(x)=f({ain A mid forall tin {rm ran}(x):t prec a})$ if $x$ is a function and ${ain A mid forall tin {rm ran}(x):t prec a} neq emptyset$, and $G(x)=A$ otherwise. By Transfinite Recursion Theorem, there is a function $F: {rm Ord} to V$ such that $F(alpha)=G(F restriction alpha)$ for all $alpha in {rm Ord}$.
It is not hard to verify (by Hartogs number) that $F(alpha)= A$ for some ordinal $alpha$. Let $lambda=min{alpha in {rm Ord} mid F(alpha)= A}$. Then ${rm ran}(Frestriction lambda)$ is clearly a chain in $(A,preccurlyeq)$ and has an upper bound $u in A$. If $u prec bar a$ for some $bar ain A$, we have $bar ain{ain A mid forall tin {rm ran}(Frestriction lambda):t prec a} neq emptyset$ and thus $F(lambda) in A$. This contradicts the fact that $F(lambda)=A$. So $u$ is the maximal element of $A$.
proof-verification elementary-set-theory ordinals
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add a comment |
$begingroup$
Well-Ordering Principle implies Zorn's Lemma
Does my attempt look fine or contain logical flaws/gaps? Any suggestion is greatly appreciated. Thank you for your help!
My attempt:
Let $(A,preccurlyeq)$ be a partially ordered set in which every chain has an upper bound.
By Well-Ordering Principle, there is a well-ordering $preccurlyeq'$ on $A$. Let $V$ be the class of all sets and $rm Ord$ be the class of all ordinals. First, we define function $f:mathcal{P}(A)setminus{emptyset} to A$ by $f(X)=min X$ (with regard to $preccurlyeq'$).
Next, we define function $G:V to V$ by $G(x)=f({ain A mid forall tin {rm ran}(x):t prec a})$ if $x$ is a function and ${ain A mid forall tin {rm ran}(x):t prec a} neq emptyset$, and $G(x)=A$ otherwise. By Transfinite Recursion Theorem, there is a function $F: {rm Ord} to V$ such that $F(alpha)=G(F restriction alpha)$ for all $alpha in {rm Ord}$.
It is not hard to verify (by Hartogs number) that $F(alpha)= A$ for some ordinal $alpha$. Let $lambda=min{alpha in {rm Ord} mid F(alpha)= A}$. Then ${rm ran}(Frestriction lambda)$ is clearly a chain in $(A,preccurlyeq)$ and has an upper bound $u in A$. If $u prec bar a$ for some $bar ain A$, we have $bar ain{ain A mid forall tin {rm ran}(Frestriction lambda):t prec a} neq emptyset$ and thus $F(lambda) in A$. This contradicts the fact that $F(lambda)=A$. So $u$ is the maximal element of $A$.
proof-verification elementary-set-theory ordinals
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A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
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– DanielWainfleet
Dec 22 '18 at 2:43
add a comment |
$begingroup$
Well-Ordering Principle implies Zorn's Lemma
Does my attempt look fine or contain logical flaws/gaps? Any suggestion is greatly appreciated. Thank you for your help!
My attempt:
Let $(A,preccurlyeq)$ be a partially ordered set in which every chain has an upper bound.
By Well-Ordering Principle, there is a well-ordering $preccurlyeq'$ on $A$. Let $V$ be the class of all sets and $rm Ord$ be the class of all ordinals. First, we define function $f:mathcal{P}(A)setminus{emptyset} to A$ by $f(X)=min X$ (with regard to $preccurlyeq'$).
Next, we define function $G:V to V$ by $G(x)=f({ain A mid forall tin {rm ran}(x):t prec a})$ if $x$ is a function and ${ain A mid forall tin {rm ran}(x):t prec a} neq emptyset$, and $G(x)=A$ otherwise. By Transfinite Recursion Theorem, there is a function $F: {rm Ord} to V$ such that $F(alpha)=G(F restriction alpha)$ for all $alpha in {rm Ord}$.
It is not hard to verify (by Hartogs number) that $F(alpha)= A$ for some ordinal $alpha$. Let $lambda=min{alpha in {rm Ord} mid F(alpha)= A}$. Then ${rm ran}(Frestriction lambda)$ is clearly a chain in $(A,preccurlyeq)$ and has an upper bound $u in A$. If $u prec bar a$ for some $bar ain A$, we have $bar ain{ain A mid forall tin {rm ran}(Frestriction lambda):t prec a} neq emptyset$ and thus $F(lambda) in A$. This contradicts the fact that $F(lambda)=A$. So $u$ is the maximal element of $A$.
proof-verification elementary-set-theory ordinals
$endgroup$
Well-Ordering Principle implies Zorn's Lemma
Does my attempt look fine or contain logical flaws/gaps? Any suggestion is greatly appreciated. Thank you for your help!
My attempt:
Let $(A,preccurlyeq)$ be a partially ordered set in which every chain has an upper bound.
By Well-Ordering Principle, there is a well-ordering $preccurlyeq'$ on $A$. Let $V$ be the class of all sets and $rm Ord$ be the class of all ordinals. First, we define function $f:mathcal{P}(A)setminus{emptyset} to A$ by $f(X)=min X$ (with regard to $preccurlyeq'$).
Next, we define function $G:V to V$ by $G(x)=f({ain A mid forall tin {rm ran}(x):t prec a})$ if $x$ is a function and ${ain A mid forall tin {rm ran}(x):t prec a} neq emptyset$, and $G(x)=A$ otherwise. By Transfinite Recursion Theorem, there is a function $F: {rm Ord} to V$ such that $F(alpha)=G(F restriction alpha)$ for all $alpha in {rm Ord}$.
It is not hard to verify (by Hartogs number) that $F(alpha)= A$ for some ordinal $alpha$. Let $lambda=min{alpha in {rm Ord} mid F(alpha)= A}$. Then ${rm ran}(Frestriction lambda)$ is clearly a chain in $(A,preccurlyeq)$ and has an upper bound $u in A$. If $u prec bar a$ for some $bar ain A$, we have $bar ain{ain A mid forall tin {rm ran}(Frestriction lambda):t prec a} neq emptyset$ and thus $F(lambda) in A$. This contradicts the fact that $F(lambda)=A$. So $u$ is the maximal element of $A$.
proof-verification elementary-set-theory ordinals
proof-verification elementary-set-theory ordinals
edited Dec 7 '18 at 6:43
Le Anh Dung
asked Dec 7 '18 at 6:01
Le Anh DungLe Anh Dung
1,0391521
1,0391521
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A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
$endgroup$
– DanielWainfleet
Dec 22 '18 at 2:43
add a comment |
$begingroup$
A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
$endgroup$
– DanielWainfleet
Dec 22 '18 at 2:43
$begingroup$
A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
$endgroup$
– DanielWainfleet
Dec 22 '18 at 2:43
$begingroup$
A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
$endgroup$
– DanielWainfleet
Dec 22 '18 at 2:43
add a comment |
1 Answer
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The proof itself is correct (oops), but saying "clearly a chain" is not a good thing to do, at least state what you are using here like you did with Hartogs number.
Let me suggest a more intuitive proof:
First, the intuition: assuming that Zorn's lemma is false, every chain is bounded but there is no maximal element. Now we create a sequence $x_beta$ to be strictly increasing sequence. Now if we build the sequence $langle x_betamid beta<alpharangle$ the set ${x_betamid beta<alpha}$ is a chain, so it has an upper bound, $y$, and because $y$ is not maximal element there is $x_alpha$ such that $yprec x_alpha$, so add $xalpha$ to the sequence, you can keep going and exhaust the ordinal like that, which is impossible.
Now, formal proof for the above:
Let $V,(A,preccurlyeq),f$ be defined as yours, assume that $A$ doesn't have maximal element, let $x_0in A$, and $H:Vto V$ defined as followed:
If $g$ is not order preserving function from some ordinal to $A$ then $H(g)=x_0$(the Do not care case).
If $g$ is a function from $betain Ord$ to $A$ and order preserving, then $g[beta]$ is a chain, let $U$ be the set of upper bounds of $g[beta]$, then take $f(U)$, now let $U'$ be the set of elements in $A$ that are greater than $f(U)$(with respect to $prec$), by assumption $U'ne emptyset$, so $H(g)=f(U')$.
Now, by the theorem there exists unique $F:Ordto A$ such that $F(beta)=H(Frestrictionbeta)$.
Use induction to prove that $Frestriction gamma:gammato A$ is order preserving for all $gamma$, so $Frestriction gamma:gammato A$ is injective, set $gamma=mbox{Hartogs’ number}$ and you got a contradiction.
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Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
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– Le Anh Dung
Dec 22 '18 at 1:52
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@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
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– Holo
Dec 22 '18 at 12:22
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You are very welcome :)
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– Le Anh Dung
Dec 22 '18 at 12:25
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@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
add a comment |
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$begingroup$
The proof itself is correct (oops), but saying "clearly a chain" is not a good thing to do, at least state what you are using here like you did with Hartogs number.
Let me suggest a more intuitive proof:
First, the intuition: assuming that Zorn's lemma is false, every chain is bounded but there is no maximal element. Now we create a sequence $x_beta$ to be strictly increasing sequence. Now if we build the sequence $langle x_betamid beta<alpharangle$ the set ${x_betamid beta<alpha}$ is a chain, so it has an upper bound, $y$, and because $y$ is not maximal element there is $x_alpha$ such that $yprec x_alpha$, so add $xalpha$ to the sequence, you can keep going and exhaust the ordinal like that, which is impossible.
Now, formal proof for the above:
Let $V,(A,preccurlyeq),f$ be defined as yours, assume that $A$ doesn't have maximal element, let $x_0in A$, and $H:Vto V$ defined as followed:
If $g$ is not order preserving function from some ordinal to $A$ then $H(g)=x_0$(the Do not care case).
If $g$ is a function from $betain Ord$ to $A$ and order preserving, then $g[beta]$ is a chain, let $U$ be the set of upper bounds of $g[beta]$, then take $f(U)$, now let $U'$ be the set of elements in $A$ that are greater than $f(U)$(with respect to $prec$), by assumption $U'ne emptyset$, so $H(g)=f(U')$.
Now, by the theorem there exists unique $F:Ordto A$ such that $F(beta)=H(Frestrictionbeta)$.
Use induction to prove that $Frestriction gamma:gammato A$ is order preserving for all $gamma$, so $Frestriction gamma:gammato A$ is injective, set $gamma=mbox{Hartogs’ number}$ and you got a contradiction.
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Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
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– Le Anh Dung
Dec 22 '18 at 1:52
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@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
add a comment |
$begingroup$
The proof itself is correct (oops), but saying "clearly a chain" is not a good thing to do, at least state what you are using here like you did with Hartogs number.
Let me suggest a more intuitive proof:
First, the intuition: assuming that Zorn's lemma is false, every chain is bounded but there is no maximal element. Now we create a sequence $x_beta$ to be strictly increasing sequence. Now if we build the sequence $langle x_betamid beta<alpharangle$ the set ${x_betamid beta<alpha}$ is a chain, so it has an upper bound, $y$, and because $y$ is not maximal element there is $x_alpha$ such that $yprec x_alpha$, so add $xalpha$ to the sequence, you can keep going and exhaust the ordinal like that, which is impossible.
Now, formal proof for the above:
Let $V,(A,preccurlyeq),f$ be defined as yours, assume that $A$ doesn't have maximal element, let $x_0in A$, and $H:Vto V$ defined as followed:
If $g$ is not order preserving function from some ordinal to $A$ then $H(g)=x_0$(the Do not care case).
If $g$ is a function from $betain Ord$ to $A$ and order preserving, then $g[beta]$ is a chain, let $U$ be the set of upper bounds of $g[beta]$, then take $f(U)$, now let $U'$ be the set of elements in $A$ that are greater than $f(U)$(with respect to $prec$), by assumption $U'ne emptyset$, so $H(g)=f(U')$.
Now, by the theorem there exists unique $F:Ordto A$ such that $F(beta)=H(Frestrictionbeta)$.
Use induction to prove that $Frestriction gamma:gammato A$ is order preserving for all $gamma$, so $Frestriction gamma:gammato A$ is injective, set $gamma=mbox{Hartogs’ number}$ and you got a contradiction.
$endgroup$
$begingroup$
Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
$endgroup$
– Le Anh Dung
Dec 22 '18 at 1:52
$begingroup$
@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
add a comment |
$begingroup$
The proof itself is correct (oops), but saying "clearly a chain" is not a good thing to do, at least state what you are using here like you did with Hartogs number.
Let me suggest a more intuitive proof:
First, the intuition: assuming that Zorn's lemma is false, every chain is bounded but there is no maximal element. Now we create a sequence $x_beta$ to be strictly increasing sequence. Now if we build the sequence $langle x_betamid beta<alpharangle$ the set ${x_betamid beta<alpha}$ is a chain, so it has an upper bound, $y$, and because $y$ is not maximal element there is $x_alpha$ such that $yprec x_alpha$, so add $xalpha$ to the sequence, you can keep going and exhaust the ordinal like that, which is impossible.
Now, formal proof for the above:
Let $V,(A,preccurlyeq),f$ be defined as yours, assume that $A$ doesn't have maximal element, let $x_0in A$, and $H:Vto V$ defined as followed:
If $g$ is not order preserving function from some ordinal to $A$ then $H(g)=x_0$(the Do not care case).
If $g$ is a function from $betain Ord$ to $A$ and order preserving, then $g[beta]$ is a chain, let $U$ be the set of upper bounds of $g[beta]$, then take $f(U)$, now let $U'$ be the set of elements in $A$ that are greater than $f(U)$(with respect to $prec$), by assumption $U'ne emptyset$, so $H(g)=f(U')$.
Now, by the theorem there exists unique $F:Ordto A$ such that $F(beta)=H(Frestrictionbeta)$.
Use induction to prove that $Frestriction gamma:gammato A$ is order preserving for all $gamma$, so $Frestriction gamma:gammato A$ is injective, set $gamma=mbox{Hartogs’ number}$ and you got a contradiction.
$endgroup$
The proof itself is correct (oops), but saying "clearly a chain" is not a good thing to do, at least state what you are using here like you did with Hartogs number.
Let me suggest a more intuitive proof:
First, the intuition: assuming that Zorn's lemma is false, every chain is bounded but there is no maximal element. Now we create a sequence $x_beta$ to be strictly increasing sequence. Now if we build the sequence $langle x_betamid beta<alpharangle$ the set ${x_betamid beta<alpha}$ is a chain, so it has an upper bound, $y$, and because $y$ is not maximal element there is $x_alpha$ such that $yprec x_alpha$, so add $xalpha$ to the sequence, you can keep going and exhaust the ordinal like that, which is impossible.
Now, formal proof for the above:
Let $V,(A,preccurlyeq),f$ be defined as yours, assume that $A$ doesn't have maximal element, let $x_0in A$, and $H:Vto V$ defined as followed:
If $g$ is not order preserving function from some ordinal to $A$ then $H(g)=x_0$(the Do not care case).
If $g$ is a function from $betain Ord$ to $A$ and order preserving, then $g[beta]$ is a chain, let $U$ be the set of upper bounds of $g[beta]$, then take $f(U)$, now let $U'$ be the set of elements in $A$ that are greater than $f(U)$(with respect to $prec$), by assumption $U'ne emptyset$, so $H(g)=f(U')$.
Now, by the theorem there exists unique $F:Ordto A$ such that $F(beta)=H(Frestrictionbeta)$.
Use induction to prove that $Frestriction gamma:gammato A$ is order preserving for all $gamma$, so $Frestriction gamma:gammato A$ is injective, set $gamma=mbox{Hartogs’ number}$ and you got a contradiction.
edited Dec 27 '18 at 13:40
answered Dec 22 '18 at 1:33
HoloHolo
5,66721031
5,66721031
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Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
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– Le Anh Dung
Dec 22 '18 at 1:52
$begingroup$
@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
add a comment |
$begingroup$
Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
$endgroup$
– Le Anh Dung
Dec 22 '18 at 1:52
$begingroup$
@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
$begingroup$
Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
$endgroup$
– Le Anh Dung
Dec 22 '18 at 1:52
$begingroup$
Hi @Holo, we have $F(emptyset)=G(F restriction emptyset)=G(emptyset)=f({ain A mid forall tin {rm ran}(emptyset):t prec a})=f({ain A mid forall tin emptyset:t prec a})=f(A)in A.$ Could you please have a closer check?
$endgroup$
– Le Anh Dung
Dec 22 '18 at 1:52
$begingroup$
@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
@LeAnhDung sorry, you are correct ( I need to stop answering questions at 3 am). So yes, you are correct
$endgroup$
– Holo
Dec 22 '18 at 12:22
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
You are very welcome :)
$endgroup$
– Le Anh Dung
Dec 22 '18 at 12:25
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
$endgroup$
– Holo
Dec 27 '18 at 13:41
$begingroup$
@LeAnhDung I edit the answer (sorry it took me some time) and added my reasoning for why my method is more intuitive
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– Holo
Dec 27 '18 at 13:41
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$begingroup$
A different approach: Let $<_W$ be well-order of $A$. For $xin Ord,$ if $f''x={f(y):yin x}$ is a $leq$-chain in $A,$ let $f(x)$ be the $<_W$-least of the $leq$-upper bounds for $f''x$. Let $x_0$ be the least $xin Ord$ such that $exists zin Ord,(xin zland f(x)=f(z)).$ Show by Transfinite Induction that $f''x$ is a $leq$-chain for every $xin Ord.$ Show that $x_0$ is a $leq$-maximal member of $A.$..... (BTW we will have $f(z)=f(x_0)$ for every ordinal $z$ greater than $x_0.$)..... Some texts use $On$ for the class of ordinals.
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– DanielWainfleet
Dec 22 '18 at 2:43