Is this explanation of Lubin–Tate theory as a generalization of roots of unity mathematically correct?
问题内容
I am preparing a presentation and would appreciate feedback on the following explanation connecting the multiplicative group with Lubin–Tate theory.
Let $K$ be a field and consider elements $x,y\in K^\times$ near the identity $1$. Write
$$ x=1+X,\qquad y=1+Y. $$
Then the group law on $K^\times$ becomes
$$ xy=(1+X)(1+Y) =1+X+Y+XY. $$
Thus, if we define
$$ F(X,Y)=X+Y+XY, $$
we obtain the formal multiplicative group law.
The multiplication-by-$n$ map $ [n](X)=(1+X)^n-1$ is an endomorphism of $F$ as it satisfies $[n](F(X,Y))=F([n]X, [n]Y)$
The $n$-torsion points of $F$ are the zeros of $[n](X)$, namely the solutions of
$$ (1+X)^n=1. $$
Hence
$$ X=\zeta_n-1, $$
where $\zeta_n$ is an $n$-th root of unity. Therefore
$$ F[n]=\{\zeta_n-1:\zeta_n^n=1\}, $$
and
$$ K(F[n])=K(\zeta_n-1)=K(\zeta_n)=K(\mu_n). $$
Thus adjoining the $n$-torsion points of the formal multiplicative group is equivalent to adjoining the $n$-th roots of unity.
My interpretation is that Lubin-Tate theory generalizes this phenomenon: one replaces the formal multiplicative group by a Lubin-Tate formal group $F$ over $\mathcal O_K$, and replaces roots of unity by the torsion points of $F$. Adjoining these torsion points then generates the abelian extensions appearing in local class field theory.
Question. Is this interpretation mathematically accurate? In particular, is it reasonable to view Lubin--Tate theory as a generalization of the fact that roots of unity arise as torsion points of the formal multiplicative group?
Ofcourse here $K$ should be a local field.
I am asking this because a broad mathematical audience will be there, where many attendees will not be specialists in algebraic number theory or local class field theory.
回答 (1)
It's quite reasonable, in fact the multiplicative group law is a Lubin-Tate group law for the $p$-adic rationals $\mathbb{Q}_p$ (for the Frobenius power series $\phi(X) = (1+X)^p - 1$).