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Let $(X,d)$ be a second-countable, locally compact and locally path-connected metric space. Suppose that $(X,d)$ has enough rectifiable paths* (as explained at the end). $\forall p \in X$, let $\Gamma_pX$ denote the space of all rectifiable paths $\gamma$ starting at $p=\gamma(0)$ and parametrized with constant speed (the constant path is denoted as $c_p$). Define $T_pX$, the formal tangent space at $p$ , to be the space of equivalent classes in $\Gamma_pX$, where $$\gamma_1 \sim \gamma_2 \iff d(\gamma_1(t),\gamma_2(t)) \sim o(t) \text{ as } t \rightarrow 0^+$$ endowed with a metric $$\rho ([\gamma_1],[\gamma_2])=\overline{\lim}_{t \rightarrow 0^+} \dfrac{d(\gamma_1(t),\gamma_2(t))}{t}$$ This metric is well-defined since $$d(\gamma_1(t),\gamma_2(t))\, \le\, d(\gamma_1(t),p)+d(\gamma_2(t),p)\, \le\, \mathrm{len}(\gamma_1)|^t_0+\mathrm{len}(\gamma_2)|^t_0\, \le\, Ct$$ for some non-negative constant $C$ by definition. It can be shown that $T_pX$ admits a non-negative homogeneous structure: For $s \ge 0$, let $s[\gamma(t)]:=[\gamma(s \cdot t)] \subseteq T_pX$, then we have equalities similar to the ones in normed vector spaces like $s[c_p]=[c_p]$ and $\rho(s[\gamma_1],s[\gamma_2])=s \rho ([\gamma_1],[\gamma_2])$.

We can also give a global topology $\tau$ on $\bigsqcup_{p \in X}T_pX$. For any open set $U \subseteq X$ and continuous function $f:U \rightarrow \Bbb{R}^+$, let $$V(U,f)=\{ (p,[\gamma]) \ :\ p \in U,\, [\gamma] \in T_pX,\, \rho([\gamma],[c_p])<f(p) \}$$ The topology $\tau$ is generated by such $V$. We call this space the formal tangent bundle of $(X,d)$ and denote it as $TX$. There is a canonical embedding from $X$ to $TX$ by mapping $p$ to $(p,[c_p])$, which is probably a cofibration.

*:$(X,d)$ is said to have enough rectifiable paths if $\forall p,q \in X$ and for every countable nowhere-dense set $S \subseteq X$ such that $p,q$ belong to the same path-connected component of $X \backslash S$, there exists a rectifiable path $\gamma \subseteq X \backslash S$ connecting $p,q$. In particular, this property is enough to ensure that given any point and direction at the point, there will be rectifiable paths from $p$ to $q$ avoiding that direction.

Let $(X,d)$ be a second-countable, locally compact and locally path-connected metric space. Suppose that $(X,d)$ has enough rectifiable paths* (as explained at the end). $\forall p \in X$, let $\Gamma_pX$ denote the space of all rectifiable paths $\gamma$ starting at $p=\gamma(0)$ and parametrized with constant speed (the constant path is denoted as $c_p$). Define $T_pX$, the formal tangent space at $p$ , to be the space of equivalent classes in $\Gamma_pX$, where $$\gamma_1 \sim \gamma_2 \iff d(\gamma_1(t),\gamma_2(t)) \sim o(t) \text{ as } t \rightarrow 0^+$$ endowed with a metric $$\rho ([\gamma_1],[\gamma_2])=\varlimsup_{t \rightarrow 0^+} \dfrac{d(\gamma_1(t),\gamma_2(t))}{t}$$ This metric is well-defined since $$d(\gamma_1(t),\gamma_2(t))\, \le\, d(\gamma_1(t),p)+d(\gamma_2(t),p)\, \le\, \mathrm{len}(\gamma_1)|^t_0+\mathrm{len}(\gamma_2)|^t_0\, \le\, Ct$$ for some non-negative constant $C$ by definition. It can be shown that $T_pX$ admits a non-negative homogeneous structure: For $s \ge 0$, let $s[\gamma(t)]:=[\gamma(s \cdot t)] \subseteq T_pX$, then we have equalities similar to the ones in normed vector spaces like $s[c_p]=[c_p]$ and $\rho(s[\gamma_1],s[\gamma_2])=s \rho ([\gamma_1],[\gamma_2])$.

We can also give a global topology $\tau$ on $\bigsqcup_{p \in X}T_pX$. For any open set $U \subseteq X$ and continuous function $f:U \rightarrow \Bbb{R}^+$, let $$V(U,f)=\{ (p,[\gamma]) \ :\ p \in U,\, [\gamma] \in T_pX,\, \rho([\gamma],[c_p])<f(p) \}$$ The topology $\tau$ is generated by such $V$. We call this space the formal tangent bundle of $(X,d)$ and denote it as $TX$. There is a canonical embedding from $X$ to $TX$ by mapping $p$ to $(p,[c_p])$, which is actually a cofibration.

*:$(X,d)$ is said to have enough rectifiable paths if $\forall p \ne q \in X$ and for every countable nowhere-dense set $S \subseteq X$ such that $p,q$ belong to the same path-connected component of $X \backslash S$, there exists a rectifiable path $\gamma \subseteq X \backslash S$ connecting $p,q$. In particular, this property is enough to ensure that given any point and direction at the point, there will be rectifiable paths from $p$ to $q$ avoiding that direction.

Post Scripts: We can modify the definition by changing "paths" to "arcs", which may make discussions simpler.

We can also give a global topology $\tau$ on $\bigsqcup_{p \in X}T_pX$. For any open set $U \subseteq X$ and continuous function $f:U \rightarrow \Bbb{R}^+$, let $$V(U,f)=\{ (p,[\gamma]) \ :\ p \in U,\, [\gamma] \in T_pX,\, \rho([\gamma],[c_p])<f(p) \}$$ The topology $\tau$ is generated by such $V$. We call this space the formal tangent bundle of $(X,d)$ and denote it as $TX$.

We can also give a global topology $\tau$ on $\bigsqcup_{p \in X}T_pX$. For any open set $U \subseteq X$ and continuous function $f:U \rightarrow \Bbb{R}^+$, let $$V(U,f)=\{ (p,[\gamma]) \ :\ p \in U,\, [\gamma] \in T_pX,\, \rho([\gamma],[c_p])<f(p) \}$$ The topology $\tau$ is generated by such $V$. We call this space the formal tangent bundle of $(X,d)$ and denote it as $TX$. There is a canonical embedding from $X$ to $TX$ by mapping $p$ to $(p,[c_p])$, which is probably a cofibration.

However, the notion of "tangency" is intuitively not a concept measuring "smoothness". It just says that two curves do not intersect "transversely", which can be precisely described by how fast the distance between a pair of points on them decreases as the points follow curves approaching an intersection. So I came up with the following definition of a quasi-smooth manifold, and wonder: Does this definition include only smooth manifolds, or does it also include exceptional structures?

Let $(X,d)$ be a second-countable, locally compact and locally path-connected metric space. Suppose that $(X,d)$ has enough rectifiable paths (as explained at the end). $\forall p \in X$, let $\Gamma_pX$ denote the space of all rectifiable paths $\gamma$ starting at $p=\gamma(0)$ and parametrized with constant speed (the constant path is denoted as $c_p$). Define $T_pX$, the formal tangent space at $p$ , to be the space of equivalent classes in $\Gamma_pX$, where $$\gamma_1 \sim \gamma_2 \iff d(\gamma_1(t),\gamma_2(t)) \sim o(t) \text{ as } t \rightarrow 0^+$$ endowed with a metric $$\rho ([\gamma_1],[\gamma_2])=\overline{\lim}_{t \rightarrow 0^+} \dfrac{d(\gamma_1(t),\gamma_2(t))}{t}$$ This metric is well-defined since $$d(\gamma_1(t),\gamma_2(t))\, \le\, d(\gamma_1(t),p)+d(\gamma_2(t),p)\, \le\, \mathrm{len}(\gamma_1)|^t_0+\mathrm{len}(\gamma_2)|^t_0\, \le\, Ct$$ for some non-negative constant $C$ by definition. It can be shown that $T_pX$ admits a non-negative homogeneous structure: for $s \ge 0$, let $s[\gamma(t)]:=[\gamma(s \cdot t)] \subseteq T_pX$, then we have equalities similar to the ones in normed vector spaces like $s[c_p]=[c_p]$ and $\rho(s[\gamma_1],s[\gamma_2])=s \rho ([\gamma_1],[\gamma_2])$.

Finally, $(X,d)$ is said to be a quasi-smooth manifold if there exists a choice of pointwise homeomorphic $T^\delta_pX$ for every $p \in X$ such that $T^\delta_pX$ is also homeomorphic to an open neighbourhood of $p$.

We can also consider a stronger requirement: If $\forall p,q \in X$ (not necessarily different), there exists a rectifiable path $\gamma : (a-\varepsilon,b+\varepsilon) \rightarrow X$ such that $\gamma(a)=p,\gamma(b)=q$ and for any $r=\gamma(t_0) \; (t_0 \in (a,b)), [\gamma(t_0 \pm t)] \in T^\delta_rX$, then we can conclude that $T^\delta_pX$ varies "quasi-smoothly" and we will say $(X,d)$ admits a regular tangent bundle (seen as a subspace of $TX$). Would every quasi-smooth manifold $X$ with a regular tangent bundle actually be a smooth manifold?

However, the notion of "tangency" is intuitively not a concept measuring "smoothness". It just says that two curves do not intersect "transversely", which can be precisely described by how fast the distance between a pair of points on them decreases as the points follow curves approaching an intersection. So I came up with the following definition of a quasi-smooth manifold, and wonder: Does this definition include only smooth manifolds, or it also include exceptional structures?

Let $(X,d)$ be a second-countable, locally compact and locally path-connected metric space. Suppose that $(X,d)$ has enough rectifiable paths* (as explained at the end). $\forall p \in X$, let $\Gamma_pX$ denote the space of all rectifiable paths $\gamma$ starting at $p=\gamma(0)$ and parametrized with constant speed (the constant path is denoted as $c_p$). Define $T_pX$, the formal tangent space at $p$ , to be the space of equivalent classes in $\Gamma_pX$, where $$\gamma_1 \sim \gamma_2 \iff d(\gamma_1(t),\gamma_2(t)) \sim o(t) \text{ as } t \rightarrow 0^+$$ endowed with a metric $$\rho ([\gamma_1],[\gamma_2])=\overline{\lim}_{t \rightarrow 0^+} \dfrac{d(\gamma_1(t),\gamma_2(t))}{t}$$ This metric is well-defined since $$d(\gamma_1(t),\gamma_2(t))\, \le\, d(\gamma_1(t),p)+d(\gamma_2(t),p)\, \le\, \mathrm{len}(\gamma_1)|^t_0+\mathrm{len}(\gamma_2)|^t_0\, \le\, Ct$$ for some non-negative constant $C$ by definition. It can be shown that $T_pX$ admits a non-negative homogeneous structure: For $s \ge 0$, let $s[\gamma(t)]:=[\gamma(s \cdot t)] \subseteq T_pX$, then we have equalities similar to the ones in normed vector spaces like $s[c_p]=[c_p]$ and $\rho(s[\gamma_1],s[\gamma_2])=s \rho ([\gamma_1],[\gamma_2])$.

Finally, $(X,d)$ is said to be a quasi-smooth manifold if there exists a choice of pointwise homeomorphic $T^\delta_pX$ for every $p \in X$ such that each $T^\delta_pX$ is also homeomorphic to an open neighbourhood of $p$. This is a subtle requirement, since if we drop the condition that $T^\delta_pX$ are pointwise homeomorphic, non-manifold cases do come into sight such as the Berkovich spaces of non-Archimedean normed fields. Moreover, in these weak-examples the total space $X$ can be partitioned into a finite collection of subsets, each admitting a choice of pointwise homeomorphic $T^\delta_pX$. Is this phenomenon just a coincidence?

We can also consider a stronger requirement: If $\forall p,q \in X$ (not necessarily different), there exists a rectifiable path $\gamma : (a-\varepsilon,b+\varepsilon) \rightarrow X$ such that $\gamma(a)=p,\gamma(b)=q$ and for any $r=\gamma(t_0) \; (t_0 \in (a,b)), [\gamma(t_0 \pm t)] \in T^\delta_rX$, then we can conclude that $T^\delta_pX$ varies "quasi-smoothly" and we will say $(X,d)$ admits a regular tangent bundle (seen as a subspace of $TX$). Would every quasi-smooth manifold $X$ with a regular tangent bundle actually be a smooth manifold?