# 'Smallest' subfield of the Surreals which is isomorphic to the Surreals as an ordered group

What is the smallest subfield $F\subset N_0$ such that $$(F,+,\times,\leq)\ncong(N_0,+,\times,\leq)$$ but $$(F,+,\leq)\cong(N_0,+,\leq)?$$ Since these are all going to be proper classes cardinality is not sufficient to delineate what we mean by 'smallest', so we use the following notion. For two proper class sized ordered fields $F,F'$ we will say that $F$ is smaller than $F'$ iff there exists an injective order preserving homomorphism of the natural value group of $F$ into the natural value group of $F'$, but the reverse does not hold.

I asked this question to a friend recently and he pointed out that the class of all Conway normal forms with rational coefficients satisfies the above isomorphism identities, and since the natural value group of an ordered field is determined by the multiplicative structure of the field I believe this means that we will have such a one sided injection between their value classes. He also pointed out that the group $\mathbb{J}$ of all purely infinite surreal numbers is the classical example of a subgroup of $N_0$ which is isomorphic to it as an ordered group but not as a field, but this structure does not form a field as it does not contain $1$.

Are there any subfields 'smaller' than this with the desired property? Can we determine what the 'smallest' one is up to an appropriate type of morphism?

• I am not sure that this is the best possible notion of 'smaller' for this question -- suggestions are appreciated. Jun 2, 2018 at 8:31
• I wrote JJ because on my text editor it looked most like $\mathbb{J}$, which is the real usual notation. The group $\mathbb{J}$ is stable under product but not does not contain $1$. Jun 3, 2018 at 10:10
• @nombre I had assumed that, for example, $\omega$ and $\frac{1}{\omega}$ were both in $\mathbb{J}$, no? Jun 3, 2018 at 10:21
• @nombre Ah, nevermind, I completely misread the definition of $\mathbb{J}$ in your email. Thank you again haha. Jun 3, 2018 at 10:25

$\DeclareMathOperator{\Noo}{\mathbf{No}}$This might actually be a dead end.
This is because if $F$ is isomorphic as an ordered group to $\Noo$, then their value classes under natural ordered group valuation, that is, the underlying orders of their value groups under natural ordered field valuation, must be isomorphic. So the value group $(v F,+,\leq)$ of $F$ is isomorphic as a linear order to $\Noo$. If it is additionnaly divisible, then it is saturated as an ordered group, hence isomorphic to $(\Noo,+,\leq)$ - possibly by a different isomorphism. Thus in general $(\mathbb{Q}.vF,+,\leq)$ and $(\Noo,+,\leq)$ are ismorphic. Of course $F$ could still be "much smaller" than $\Noo$, for instance it might be the field generated by all monomials. I don't know if this is the type of field you are looking for.
• @JoelDavidHamkins: Yes, maybe "hence isomorphic to" is a little quick. Although here it must not be too difficult to recover a bijection $\mathbb{Q}. v F \rightarrow \mathbf{Ord}$ from the given group isomorphism. Jun 5, 2018 at 5:56