It is not clear to me if the disjoint union of a $d_{1}$-manifold and a $d_{2}$-manifold is still a manifold?
123430
Can a topological manifold be non-connected and each component with different dimension?
90365
Riemannian Manifolds with $n(n+1)/2$ dimensional symmetry group
91777
Product of Non-Disjoint Cycles
148685
Equivalences of definitions of submanifolds in $\mathbb{R^n}$
99465
Is every Lie subgroup of $SU(2)$ closed?
89649
Linear Algebra munkres analysis on manifolds question.
99230
If a manifold M has zero Euler characteristic, there is a non-vanishing vector field on it
127996
$M_1$ and $M_2$ are subgroups and $M_1/N=M_2/N$. Is $M_1\cong M_2$?
18687
First and second homotopy groups of a connected Lie group
93471
Space of homeomorphisms Homeo$(S^1)$ of $S^1$ deformation retracts onto $O(2)$
21198
Proving that the general linear group is a differentiable manifold
139922
Is it known what the necessary and sufficient conditions are for the existence of a "3+1 split" (by means of a foliation) of a (Lorentzian) manifold?
84419
Product of manifolds & orientability
79711
How to show $\omega^n$ is a volume form in a symplectic manifold $(M, \omega)$?
128410
Showing that the set of fixed points is a smooth manifold
16860
set of almost complex structures on $\mathbb R^4$ as two disjoint spheres
99755
Non-orientable 1-dimensional (non-hausdorff) manifold