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By embedding here, I mean merely a proper class isomorphism from

Set theorists quite commonly consider

This embedding concept is considerably weaker than usually considered in set theory, where one typically has embeddings with that are at least a small degree of elementarity, and for such kind of embeddings, the question has a strong negative answer. There can be noelementary embedding $j:V\to L$ when $V\neq L$, and indeed\Delta_0$-elementary if not much more. Of course, therecan be no we may easily refute the existence of nontrivial fully elementary or even of$\Delta_0$-elementary embedding embeddings$j:V\to L$. So the embeddings of the question will necessarily exhibit very little elementarity. MeanwhileThose arguments, it is easy however, simply fail with this much weaker embedding concept. One can begin to see this by observing that if$V=L$, then $$j(x)=\{\\{0,x\}\ \}$$ is a nontrivial defines an embedding$j:L\to L$with$j(x)\neq x$for every$x$. In particular, and there aremany other similar such embeddings from the existence of a nontrivial embedding$j:L\to L$to in this weak sense is consistent with$L$, V=L$ and these carry carries no large cardinal strength, and do does not imply prove the existence of $0^\sharp$.

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Main Question. Can there be an embedding $j:V\to L$ of the set-theoretic universe $V$ to the constructible universe $L$, if $V\neq L$?

By embedding here, I mean a proper class isomorphism from $\langle V,{\in}\rangle$ to its range in $\langle L,{\in}\rangle$, or in other words a quantifier-free-elementary map $j:V\to L$, a class map $j$ for which $x\in y\iff j(x)\in j(y)$.

Set theorists quite commonly consider embeddings with at least a small degree of elementarity, and for such kind of embeddings, the question has a strong negative answer. There can be no elementary embedding $j:V\to L$ when $V\neq L$, and indeed, there can be no nontrivial $\Delta_0$-elementary embedding $j:V\to L$. So the embeddings of the question will necessarily exhibit very little elementarity. Meanwhile, it is easy to see that if $V=L$, then $$j(x)=\{\ j(y)\mid y\in x\ \}\cup\{\ \{0,x\}\ \}$$ is a nontrivial embedding $j:L\to L$, and there are many other similar such embeddings from $L$ to $L$, and these carry no large cardinal strength and do not imply the existence of $0^\sharp$.

The question arises in connection with my paper,

where it appears in the final section with the other questions I ask here, among others. I have half an expectation, a gnawing suspicion, however, that this questions may admit an easy answer, and this is why I am asking it here. But I don't know which way the answer will go.

The main theorem of the paper shows that every countable model of set theory $M$ has an embedding $j:M\to L^M$. But the proof establishes the existence of such embeddings only in an external way, using the countability of $M$. The main question above inquires from an internal perspective whether one can ever find such an embedding as a class inside the model.

The existence of such an embedding as a definable class would of course imply $V=HOD$, since one could pull back the canonical order from $L$ to $V$. More generally, if $j$ is merely a class in Gödel-Bernays set theory, then the existence of an embedding $j:V\to L$ implies global choice. So we cannot expect every model of ZFC or of GB to have such embeddings. Can they be added generically? Do they have some large cardinal strength? Are they outright refutable?

There are several more concrete versions of the question.

Question. Does every set $A$ admit an embedding $j:\langle A,{\in}\rangle \to \langle L,{\in}\rangle$? If not, which sets do admit such embeddings?

It follows from the main theorem of the paper that every countable set $A$ embeds into $L$. What about uncountable sets?

Question. Does $\langle V_{\omega+1},{\in}\rangle$ embed into $\langle L,{\in}\rangle$? How about $\langle P(\omega),{\in}\rangle$ or $\langle \text{HC},{\in}\rangle$?

These latter questions are interesting principally when $V$ has non-constructible reals. I would be very interested in learning the answer.

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# Can there be an embedding j:V to L, from the set-theoretic universe V to the constructible universe L, when V not= L?

Main Question. Can there be an embedding $j:V\to L$ of the set-theoretic universe $V$ to the constructible universe $L$, if $V\neq L$?

By embedding here, I mean a proper class isomorphism from $\langle V,{\in}\rangle$ to its range in $\langle L,{\in}\rangle$, or in other words a quantifier-free-elementary map $j:V\to L$, a class map $j$ for which $x\in y\iff j(x)\in j(y)$.

Set theorists quite commonly consider embeddings with at least a small degree of elementarity, and for such kind of embeddings, the question has a strong negative answer. There can be no elementary embedding $j:V\to L$ when $V\neq L$, and indeed, there can be no nontrivial $\Delta_0$-elementary embedding $j:V\to L$. So the embeddings of the question will necessarily exhibit very little elementarity. Meanwhile, it is easy to see that if $V=L$, then $$j(x)=\{\ j(y)\mid y\in x\ \}\cup\{\ \{0,x\}\ \}$$ is a nontrivial embedding $j:L\to L$, and there are many other similar such embeddings from $L$ to $L$, and these carry no large cardinal strength and do not imply the existence of $0^\sharp$.

The question arises in connection with my paper,

where it appears in the final section with the other questions I ask here, among others. I have half an expectation, a gnawing suspicion, however, that this questions may admit an easy answer, and this is why I am asking it here. But I don't know which way the answer will go.

The main theorem of the paper shows that every countable model of set theory $M$ has an embedding $j:M\to L^M$. But the proof establishes the existence of such embeddings only in an external way, using the countability of $M$. The main question above inquires from an internal perspective whether one can ever find such an embedding as a class inside the model.

The existence of such an embedding as a definable class would of course imply $V=HOD$, since one could pull back the canonical order from $L$ to $V$. More generally, if $j$ is merely a class in Gödel-Bernays set theory, then the existence of an embedding $j:V\to L$ implies global choice. So we cannot expect every model of ZFC or of GB to have such embeddings. Can they be added generically? Do they have some large cardinal strength? Are they outright refutable?

There are several more concrete versions of the question.

Question. Does every set $A$ admit an embedding $j:\langle A,{\in}\rangle \to \langle L,{\in}\rangle$? If not, which sets do admit such embeddings?

It follows from the main theorem of the paper that every countable set $A$ embeds into $L$. What about uncountable sets?

Question. Does $\langle V_{\omega+1},{\in}\rangle$ embed into $\langle L,{\in}\rangle$? How about $\langle P(\omega),{\in}\rangle$ or $\langle \text{HC},{\in}\rangle$?

I would be very interested in learning the answer.