I have the following naive (and inexpert) question about the
reduction of Shimura curves at primes dividing the discriminant
of the underlying quaternion algebra. It requires some background
to state. That is, let $F$ be a totally real field of degree $d$.
Fix a real place $\tau\_1$ in the set of real
places $\lbrace \tau\_1, \ldots, \tau\_d \rbrace$ of $F$. Let
$B$ be a quaternion algebra over $F$ that is split at $\tau\_1$
and ramified at $\tau\_2, \ldots, \tau\_d$. Let $H \subset 
\widehat{B}~(= B \otimes \widehat{Z})$ be a compact open
subgroup. Let $M\_H$ denote the Shimura curve over $F$ of
level $H$, with complex points given by \begin{align\*}
M\_H({\bf{C}}) &= B^{\times}\backslash \widehat{B}^{\times} 
\times \left({\bf{C}}-{\bf{R}} \right)/H.\end{align\*}
Fix a prime $v \subset \mathcal{O}\_{F}$. Assume that
$H$ can be factored as $H^v \times H\_v$, with $H\_v \subset 
B\_v^{\times}~(= B^{\times} \otimes F\_v)$ maximal. If $v$ does
not divide the discriminant of $B$, then it is known by
work of Morita and Carayol that $M\_H$ has good reduction
over $v$, hence that there exists a smooth model ${\bf{M}}\_H$
of $M\_H$ over $\mathcal{O}\_{(v)}$. If $v$ divides the discriminant
of the quaternion algebra $B$, then it is known by work of Varshavsky
for instance that there exists an integral model
${\bf{M}}\_{H}^V$ of $M\_H$ over $\mathcal{O}\_{(v)}$. (N.B. there
is apparently also a model due to Drinfeld, described extensively
in the literature for the case of $F={\bf{Q}}$, though it
is not clear to me why Drinfeld's work, which seems
to require a moduli theoretic description of $M\_H$, extends
to the general totally real fields setting). Anyhow, let
$F\_v$ denote the completion of $F$ at $v$, with $\kappa\_v$ the
residue field and $\pi\_v$ a uniformizer. By
Cerednik's theorem, the completion of ${\bf{M}}\_H^V$
along its closed fibre is canonically isomorphic to the product
\begin{align\*} GL(F\_v)\backslash \widehat{\Omega}^{unr} \times 
D^{\times}\backslash \widehat{D}^{\times}/\overline{H}^v.
\end{align\*} Here, $\widehat{\Omega}^{unr}$ denotes the product
$\widehat{\Omega} \times\_{\operatorname{Spf}\mathcal{O}\_{F\_v}} \operatorname{Spf} 
\mathcal{O}\_{F\_v}^{unr}$, where $\widehat{\Omega}$ denotes the $v$-adic
upper half plane (viewed as a formal scheme), and
$\mathcal{O}\_{F\_v}^{unr}$ the ring of Witt vectors with coefficients
in $\overline{\kappa}\_v$. The action of $\gamma \in GL(F\_v)$ on
$\widehat{\Omega}^{unr}$ is via the image of $\gamma$ in $PGL(F\_v)$
on the component $\widehat{\Omega}$, and via multiplication by
$\operatorname{Frob}\_v^{n(\gamma)}$ on
$\widehat{\mathcal{O}}\_{F\_v}^{unr}$, where $n(\gamma) = - 
ord\_v \left( \det(\gamma) \right)$. As well,
$D$ denotes the totally definite quaternion algebra over $F$ obtained
from $B$ by switching invariants at $v$ and $\tau\_1$, with
$\overline{H}^v$ the compact open subgroup of $\widehat{D}^{\times v}$
corresponding to $H^v$ under a fixed isomorphism $B^{\times v} \cong 
D^{\times v}$. The theory of Mumford–Kurihara unifomization then gives
the following information about this curve ${\bf{M}}\_{H}^V$:


1. The curve ${\bf{M}}\_{H}^V$ is an *admissible* curve
over $\mathcal{O}\_{F\_v}$ in the sense of Jordan–Livne, i.e.


(i) ${\bf{M}}\_H^V$ is a flat, proper curve over $\mathcal{O}\_{F\_v}$
with a smooth generic fibre.


(ii) The special fibre of ${\bf{M}}\_H^V$ is reduced; the normalization
of each of its irreducible components is isomorphic to ${\bf{P}}^1\_{\kappa\_v}$,
and its only singular points are $\kappa\_v$-rational, ordinary double points.


(iii) The local ring ${\bf{M}}\_{H, x}$ at any singular point $x$ of
the special fibre is isomorphic as an $\mathcal{O}\_{F\_v}$-algebra to
$\mathcal{O}\_{F\_v}[[X,Y]]/(XY - \pi\_v^{m(x)})$, for $m(x) \geq 1$ a uniquely
determined integer.
2. The dual graph $\mathcal{G}({\bf{M}}\_H^V)
 = (\mathcal{V}({\bf{M}}\_H^V),\mathcal{E}({\bf{M}}\_H^V))$
of the special fibre of ${\bf{M}}\_H^V$ is  

isomorphic to $GL(F\_v)^{+} \backslash \left( 
\Delta \times D^{\times}\backslash \widehat{D}^{\times}/\overline{H}^v
\right)$, minus any loops. Here, $GL(F\_v)^+ \subset GL(F\_v)$ denotes the
collection of matrices with determinant having even $v$-adic valuation,
and $\Delta = (\mathcal{V}(\Delta), \mathcal{E}(\Delta))$ the
Bruhat–Tits tree of $SL(F\_v)$.


My question is the following: why is the edgeset
$\mathcal{E}({\bf{M}}\_H^V)$ nonempty? The dual graph
$\mathcal{G}({\bf{M}}\_H^V)$ is clearly disconnected, and
seen easily to be given by the disjoint union of connected
graphs \begin{align\*} \coprod\_i \mathcal{G}\_i &= 
\coprod\_i \overline{\Gamma}\_i \backslash \Delta.
\end{align\*} Here, each $\overline{\Gamma}\_i$ denotes
the image in $PGL(F\_v)$ of a suitable arithmetic subgroup
$\Gamma\_i \subset D^{\times} \cong D\_v^{\times} \cong GL(F\_v)$.
Each component graph $\mathcal{G}\_i = (\mathcal{V}\_i,
\mathcal{E}\_i)$ is connected. Now, since $\Delta$ is a tree,
each component graph $\mathcal{G}\_i = \overline{\Gamma}\_i 
\backslash \Delta$ is a tree. It is then well known that
each (first) Betti number $\beta(\mathcal{G}\_i) := 
\vert \mathcal{E}\_i \vert - \vert \mathcal{V}\_i \vert + 1$ must
vanish, i.e. $\vert \mathcal{E}\_i \vert = 
\vert \mathcal{V}\_i \vert -1 = 0$. If so, then the cardinality
of the edgeset $\mathcal{E}({\bf{M}}\_H^V)$ must also equal
zero. i.e. the special fibre of ${\bf{M}}\_H^V$ would have no
singular points ... what have I missed here?



"Bad" reduction of Shimura curves via dual graphs
[inkspot](https://mathoverflow.net/questions/51935/bad-reduction-of-shimura-curves-via-dual-graphs/51947#51947) is indeed correct that the component graphs are indeed not generally trees.


As you seem to have deduced for yourself, Cerednik–Drinfeld uniformization is a highly nontrivial concept, and it really helps to have some examples to set it in your mind. Most helpful in this direction is Ogg's "Mauvaise réduction des courbes de Shimura" ([Zbl 0581.14024](https://zbmath.org/?q=an:0581.14024 "Théorie des nombres, Sémin. Delange-Pisot-Poitou, Paris 1983-84, Prog. Math. 59, 199-217 (1985). Book available at https://link.springer.com/book/9780817642594")) where he draws out a few of these dual graphs.


In general the dual graphs have $2h$ vertices $x$ where $h$ is the class number of $\mathcal{O}\_x$, a level $H$ Eichler order in $D$ (your totally definite quaternion algebra, so note there's a choice of which $x$ to make here, but as long as the level is squarefree all orders are hereditary and it doesn't make a difference).


An orbit-stabilizer theorem computation then shows that for instance when $B$ is a quaternion algebra over $\mathbf{Q}$, $p+1$ (the size of the set of edges $y$ stemming from a particular vertex $x$ in the Bruhat–Tits tree) is equal to $\sum\_{e(y) = x} \frac{\mathcal{O}\_x^\times}{\mathcal{O}\_y^\times}$. So it's not just that there are edges, but we know exactly how many there are! (For a readable account of details of this, see Kurihara's paper on [Equations defining Shimura Curves](https://doi.org/10.15083/00039671 "Kurihara, Akira. On some examples of equations defining Shimura curves and the Mumford uniformization. J. Fac. Sci., Univ. Tokyo, Sect. I A 25, 277-300 (1978). zbMATH review at https://zbmath.org/?q=an:0428.14012").)


Also, if I may take issue with 1.(ii) and 1.(iii), you've given a good description of the special fiber over $\overline \kappa\_v$ (which is what I'm taking the dual graph to represent the data for), not necessarily $\kappa\_v$. What you've claimed is that the special fiber is a Mumford Curve, that is, the transverse union of a number of copies of $\mathbb{P}^1$'s. The truth is that the special fiber is a *quadratic twist* of a Mumford curve, where the Galois action is not simply given by the $| \kappa\_v|$-Frobenius, but where the action of Frobenius is identified with the action of the Atkin–Lehner operator $w\_p$, which interchanges some of the components (if you want to think about the graph, its vertex set can be partitioned into $ \{x\_1, \dots , x\_h, x\_1', \dots, x\_h'\}$ where $w\_p(x\_i) = x\_i'$).


All that said, some of the best advice I've heard for trying to understand this stuff is to first completely understand what happens when $v$ divides the LEVEL because in that case the moduli problem is much easier (if an abelian variety here is isogenous to a product of supersingular elliptic curves, it's isomorphic to a product of supersingular elliptic curves).


Here are a few additional references:


* Boutot–Carayol, [$p$-adic uniformization of Shimura curves: the theorems of Cerednik and Drinfeld](https://ms.mcmaster.ca/%7Efrancc13/files/bctranslation.pdf "Courbes modulaires et courbes de Shimura, C. R. Sémin., Orsay/Fr. 1987-88, Astérisque 196-197, 45-158 (1991). Available at http://www.numdam.org/item/AST_1991__196-197__1_0/. zbMATH review at https://zbmath.org/?q=an:0781.14010") (a translation by Cameron Franc)
* Kenneth A. Ribet, [Bimodules and Abelian Surfaces](https://math.berkeley.edu/%7Eribet/Articles/bimodules.pdf "Algebraic number theory - in honor of K. Iwasawa, Proc. Workshop Iwasawa Theory Spec. Values L-Funct., Berkeley/CA (USA) 1987, Adv. Stud. Pure Math. 17, 359–407 (1989), doi:10.2969/aspm/01710359. zbMATH review at https://zbmath.org/?q=an:0742.11033") (this includes a somewhat more intuitive description of the components of the Mumford curve)
* Pete L. Clark, [Rational Points on Atkin–Lehner
Quotients of Shimura Curves](http://alpha.math.uga.edu/%7Epete/thesis.pdf), Ph.D. thesis (a comprehensive introduction to Shimura Curves and the action of the Atkin–Lehner group)
* Frans Oort, [Which abelian surfaces are products of elliptic curves?](https://doi.org/10.1007/BF01428253 "Math. Ann. 214, 35–47 (1975). Available at https://eudml.org/doc/162679. zbMATH review at https://zbmath.org/?q=an:0283.14007") (see also his book on [moduli of supersingular abelian varieties](https://doi.org/10.1007/BFb0095931 "Ke-Zheng Li, Frans Oort. Moduli of supersingular abelian varieties. Lecture Notes in Mathematics. 1680. Berlin: Springer (1998). zbMATH review at https://zbmath.org/?q=an:0920.14021"), as when $v$ ramifies in $B$, you're asking a question about moduli of supersingular abelian varieties, see the appendix on Honda–Tate theory to the thesis above)


