Let $G$ be a semisimple group over $\mathbb{C}$ and let $X=G/H$ be a spherical homogeneous space,
then $X$ defines a spherical datum (Luna datum) $\mathcal L(X)=(N,\mathcal V, \mathcal D, \rho,\varsigma)$, see below.


The spherical datum is a functor. Indeed, assume we have two spherical homogeneous spaces $X\_1$ and $X\_2$ of $G$.
A $G$-morphism $\varphi\colon X\_1\to X\_2$ induces a morphism $\mathcal L(\varphi)\colon \mathcal L(X\_1)\to \mathcal L(X\_2)$ (in the obvious sense, see below).


Losev's Theorem 1 from his paper [Uniqueness property for spherical homogeneous spaces](http://arxiv.org/abs/0904.2937) says that if there exists an *isomorphism* $\lambda\colon\mathcal L(X\_1)\to \mathcal L(X\_2)$, then there exists an isomorphism
$\varphi\colon X\_1\to X\_2$.



> 
> **Question.** Assume we have two spherical homogeneous spaces $X\_1$ and $X\_2$ of $G$, and a *morphism* of spherical data 
>  $\lambda\colon \mathcal L(X\_1)\to \mathcal L(X\_2)$.
>  Does there exist a $G$-morphism $\varphi\colon X\_1\to X\_2$ *inducing* $\lambda$, that is, such that $\lambda=\mathcal L(\varphi)$?
> 
> 
> 


A positive answer to this question would strengthen Losev's theorem even in the case when $\lambda$ is an isomorphism.


We specify our version of the spherical datum of $X$.
Let $B\subset G$ be a Borel subgroup, and let $T\subset B\subset G$ be a maximal torus.
Let $S=S(G,T,B)$ denote the corresponding system of simple roots.
Let ${\mathcal{P}}(S)$ denote the set of subsets of $S$.


Let ${\mathcal{X}}(B)$ denote the character group of $B$, and let $M\subset {\mathcal{X}}(B)$ denote the weight lattice of $X$.
Set $N={\rm Hom}(M,{\mathbb{Z}})$, $N\_{\mathbb{Q}}=N\otimes\_{\mathbb Z}{\mathbb{Q}}$.
Let $\mathcal{V}\subseteq N\_{\mathbb{Q}}$ denote the valuation cone of $X$.


Let ${\mathcal{D}}$ denote the set of colors of $X$ (the set of $B$-invariant prime divisors of $X$). We have two maps:
$$\rho\colon {\mathcal{D}}\to N,\qquad \varsigma\colon {\mathcal{D}}\to {\mathcal{P}}(S). $$
Here, for $D\in{\mathcal{D}}$,
$$ \varsigma(D)=\{\alpha\in S\ | \ P\_\alpha\cdot D\neq D\}, $$
where $P\_\alpha\supset B$ denotes the parabolic subgroup corresponding to $\alpha\in S$.
We set $\mathcal L(X)=(N,\mathcal V, \mathcal D, \rho,\varsigma)$.


If $\mathcal L(X\_i)=(N\_i,\mathcal V\_i,\mathcal D\_i, \rho\_i,\varsigma\_i)$ for $i=1,2$,
then a $G$-morphism $\varphi\colon X\_1\to X\_2$ induces a morphism of spherical data
$$ \mathcal L(\varphi)=(\ \lambda\_N\colon N\_1\to N\_2,\quad \lambda\_{\mathcal D}\colon\mathcal D\_1\to\mathcal D\_2\ ) $$
such that $(\lambda\_N\otimes\_{\mathbb Z} \mathbb Q)(\mathcal V\_1)\subseteq \mathcal V\_2$ and the corresponding diagrams for $\varsigma\_i$ and for $\rho\_i$ commute: for any $D\_1\in\mathcal D\_1$ we have
$$\lambda\_N(\rho\_1(D\_1))=\rho\_2(\lambda\_{\mathcal D}(D\_1)),\qquad
 \varsigma\_1(D\_1)=\varsigma\_2(\lambda\_{\mathcal D}(D\_1)). $$
This permits us to define an abstract morphism of spherical data $\lambda\colon \mathcal L(X\_1)\to\mathcal L(X\_2)$ and to ask our question above.



Morphisms of the spherical data of spherical homogeneous spaces
The situation is more complicated. First of all $\lambda\_{\mathcal D}$ does not exist, since there may be colors in $X\_1$ which map dominantly to $X\_2$. The best way to describe morphisms $X\_1\to X\_2$ is to use some kind of Stein factorization $X\_1\to X'\to X\_2$ where $X\_1\to X'$ has connected fibers and $X'\to X\_2$ is a finite morphism. If $X\_i=G/H\_i$ then $X'=G/H'$ with $H'=H\_1H\_2^0$. The finite morphisms are just quotients by finite automorphism groups and are therefore handled by Losev's theorem on the automorphism group of spherical varieties.


The morphisms with connected fibers were described in my Hyderabad paper. They are classified by so-called colored subspaces. More precisely, let $\mathcal D^0\subseteq \mathcal D\_1$ be the set of colors mapping dominantly to $X\_2$. Let $N^0\subseteq N\_1$ be the kernel of $\lambda\_N$. Then $\mathcal D^0$ is an additional datum to $\lambda$ and $\lambda\_{\mathcal D}$ is only defined on $\mathcal D':=\mathcal D\_1\setminus\mathcal D^0$. The requirement is that $(N^0,\mathcal D^0)$ is a colored subspace, meaning that $N^0$ is generated as a convex cone by $N^0\cap\mathcal V\_1$ and $\rho\_1(\mathcal D^0)$.


So assembling both cases one gets: A spherical morphism is a pair $(\lambda\_N,\lambda\_{\mathcal D})$ where $\lambda\_N$ is a homomorphism $N\_1\to N\_2$ and $\lambda\_{\mathcal D}$ is a map defined on a subset $\mathcal D'$ of $\mathcal D\_1$ to $\mathcal D\_2$. The axioms are roughly


* $\rho\_2\circ\lambda\_{\mathcal D}(D)=\lambda\_N\circ\rho\_1(D)$ and $\varsigma\_2\circ\lambda\_{\mathcal D}(D)=\varsigma\_1(D)$ for all $D\in{\mathcal D'}$
* $\lambda\_N^{\mathbb Q}(\mathcal V\_1)=\mathcal V\_2$
* $(\ker\lambda\_N^{\mathbb Q},\mathcal D\_1\setminus\mathcal D')$ is a colored subspace.
* The spherical datum $(N\_2,\mathcal V\_2,\mathcal D\_2)$ is obtained from the spherical datum $(\lambda\_N(N\_1),\mathcal V\_2,\mathcal D')$ according to Losev's rules, i.e., certain spherical roots are doubled and certain colors are identified in an admissible way.


**Edit:** I just realized that a morphism between spherical data does not uniquely determine the morphism between spherical varieties. For example a twist by a central element of $G$ is not seen on the level of spherical data.


