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Let $T(S)$ be the Teichmuller space over a hyperbolic Riemann surface $S$. A geodesic disk in $T(S)$ is defined the image of an isometric embedding of the Poincar\'e disk into $T(S)$. In this paper, it is shown that for any non-Strebel point $\tau\in T(S)\backslash\{[0]\}$, there are infinitely many geodesic disks containing the straight line $\{[t\mu]:\,t\in (-1/k,1/k)\}$ where $\mu$ is an extremal representative of $\tau$ with $\|\mu\|_\infty=k$. An infinitesimal version is also obtained.

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In this paper, several versions of gluing theorems for quasiconformal mappings in the plane are obtained. The best possibility of gluing quasiconformal mappings is investigated. As an application, we provide a new short proof of the gluing theorem obtained by Jiang and Qi.

We study rooted cluster algebras and rooted cluster mor-phisms which were introduced in [1]recently and cluster struc-tures in 2-Calabi–Yau triangulated categories. An example of rooted cluster morphism which is not ideal is given, this clar-ifyinga doubt in [1]. We introduce the notion of freezing of a seed and show that an injective rooted cluster morphism always arises from a freezing and a subseed. Moreover, it is a section if and only if it arises from a subseed. This answers the Problem 7.7 in [1]. We prove that an inducible rooted clus-ter morphism is ideal if and only if it can be decomposed as a surjective rooted cluster morphism and an injective rooted cluster morphism. For rooted cluster algebras arising from a 2-Calabi–Yau triangulated category Cwith cluster tilting ob-jects, we give an one-to-one correspondence between certain pairs of their rooted cluster subalgebras which we call com-plete pairs (see Definition2.27) and cotorsion pairs in C.