Split up the first section into several smaller sections.

config/en/mkdocs.yaml

@@ -97,6 +97,8 @@ nav:
         - 'Tensor symmetries': mathematics/linear-algebra/tensors/tensor-symmetries.md
         - 'Volume forms': mathematics/linear-algebra/tensors/volume-forms.md
         - 'Tensor transformations': mathematics/linear-algebra/tensors/tensor-transformations.md
+      - 'Topology':
+        - 'Fiber bundles': mathematics/topology/fiber-bundles.md
     - 'Calculus': 
       - 'Limits': mathematics/calculus/limits.md
       - 'Continuity': mathematics/calculus/continuity.md
@@ -125,6 +127,8 @@ nav:
       - 'The Laplace transform': mathematics/ordinary-differential-equations/laplace-transform.md
     - 'Differential geometry':
       - 'Differential manifolds': mathematics/differential-geometry/differential-manifolds.md
+      - 'Tangent spaces': mathematics/differential-geometry/tangent-spaces.md
+      - 'Transformations': mathematics/differential-geometry/transformations.md
       - 'Linear connections': mathematics/differential-geometry/linear-connections.md
       - 'Derivatives': mathematics/differential-geometry/derivatives.md
       - 'Torsion': mathematics/differential-geometry/torsion.md

docs/en/mathematics/differential-geometry/differential-manifolds.md

@@ -1,6 +1,6 @@
 # Differential manifolds
 
-In the following sections we make use of the Einstein summation convention introduced in [vector analysis](/en/physics/mathematical-physics/vector-analysis/curvilinear-coordinates/) and $\mathbb{K} = \mathbb{R}$ or $\mathbb{K} = \mathbb{C}.$
+In the following sections of differential geometry we make use of the Einstein summation convention introduced in [vector analysis](/en/physics/mathematical-physics/vector-analysis/curvilinear-coordinates/) and $\mathbb{K} = \mathbb{R}$ or $\mathbb{K} = \mathbb{C}.$
 
 ## Definition
 
@@ -16,9 +16,9 @@ Differential geometry is concerned with *differential manifolds*, smooth continu
 
 The last axiom ensures that any chart is tacitly assumed to be already contained in the atlas.
 
-## Transformations
+## Coordinate transformations
 
-> *Definition 2*: let $p,q \in \mathrm{M}$ be points on the differential manifold and let $\psi: \mathscr{D}(\psi) \to\mathrm{M}: p \mapsto \psi(p) \overset{\text{def}}{=} q$ be a **transformation** on the manifold, we define two diffeomorphisms
+> *Definition 2*: let $p,q \in \mathrm{M}$ be points on the differential manifold and let $\psi: \mathscr{D}(\psi) \to\mathrm{M}: p \mapsto \psi(p) \overset{\text{def}}{=} q$ be a **transformation** from $p$ to $q$ on the manifold, we define two diffeomorphisms
 > 
 > $$
 >   \phi_\alpha: \mathscr{D}(\phi_\alpha) \to \mathscr{R}(\phi_\alpha): p \mapsto \phi_\alpha(p) \overset{\text{def}}{=} x,
@@ -38,191 +38,4 @@ The last axiom ensures that any chart is tacitly assumed to be already contained
 
 To clarify the definitions, a passive transformation corresponds only to a descriptive transformation. Whereas an active transformation corresponds to a transformation on the manifold $M$.  
 
-A passive transformation may also be given directly by $\phi_\beta \circ \phi_\alpha: x \mapsto y$ since $\psi = \mathrm{id}$ in this case. Note that the definitions could also have been given by the inverse as the transformations are all diffeomorphisms. 
+A passive transformation may also be given directly by $\phi_\beta \circ \phi_\alpha: x \mapsto y$ since $\psi = \mathrm{id}$ in this case. Note that the definitions could also have been given by the inverse as the transformations are all diffeomorphisms. 
-
-## Fiber bundles
-
-(This subsection should probably be moved to a more general setting of manifolds.)
-
-> *Definition 3*: a **fiber** $V_x$ at a point $x \in \mathrm{M}$ on a  manifold is a finite dimensional vector space. With the collection of fibers $V_x$ for all $x \in \mathrm{M}$ define the **fiber bundle** as
->
-> $$
->   V = \bigcup_{x \in \mathrm{M}} V_x.
-> $$
-
-Then by definition we have the projection map $\pi$ given by
-
-$$
-    \pi: V \to\mathrm{M}: (x,\mathbf{v}) \mapsto \pi(x, \mathbf{v}) \overset{\text{def}}{=} x,
-$$
-
-and its inverse
-
-$$
-    \pi^{-1}:\mathrm{M} \to V: x \mapsto \pi(x) \overset{\text{def}}{=} V_x.
-$$
-
-Similarly, a dual fiber $V_x^*$ may be defined for $x \in \mathrm{M}$, with its fiber bundle defined by
-
-$$
-    V^* = \bigcup_{x \in \mathrm{M}} V_x^*.
-$$
-
-> *Definition 4*: a **tensor fiber** $\mathscr{B}_x$ at a point $x \in \mathrm{M}$ on a manifold is defined as
->
-> $$
->   \mathscr{B}_x = \bigcup_{p,q \in \mathbb{N}} \mathscr{T}^p_q(V_x).
-> $$
->
-> With the collection of tensor fibers $\mathscr{B}_x$ for all $x \in \mathrm{M}$ define the **tensor fiber bundle** as
->
-> $$
->   \mathscr{B} = \bigcup_{x \in \mathrm{M}} \mathscr{B}_x.
-> $$
-
-Then for a point $x \in \mathrm{M}$ we have a tensor $\mathbf{T} \in \mathscr{B}_x$ such that
-
-$$
-    \mathbf{T} = T^{ij}_k \mathbf{e}_i \otimes \mathbf{e}_j \otimes \mathbf{\hat e}^k, 
-$$
-
-with $T^{ij}_k \in \mathbb{K}$ holors of $\mathbf{T}$. Furthermore, $\{\mathbf{e}_i\}_{i=1}^n$ a basis of $V_x$ and $\{\mathbf{\hat e}^i\}_{i=1}^n$ a basis of $V_x^*$. 
-
-> *Definition 5*: a tensor field $\mathbf{T}$ on a manifold $M$ is a [section]()
->
-> $$
->   \mathbf{T} \in \Gamma(\mathrm{M}, \mathscr{B}),
-> $$
->
-> of the tensor fiber bundle $\mathscr{B}$. 
-
-Therefore, a tensor field assigns a tensor fiber (or tensor) to each point on a section of the  manifold. These tensors may vary smoothly along the section of the manifold.
-
-## Tangent bundles
-
-> *Definition 6*: let $f \in C^{\infty}(\mathrm{M})$ with $C^{\infty}$ the class of [smooth functions]() and $M$ a differential manifold. A derivation of $f$ at $x \in \mathrm{M}$ is defined as a linear map $\mathbf{v}_x: C^\infty(\mathrm{M}) \to \mathbb{K}$ that satisfies
->
-> $$
->   \forall f,g \in C^{\infty}(\mathrm{M}): \mathbf{v}_x(f g) = (\mathbf{v}_xf) g + f (\mathbf{v}_x g).
-> $$
->
-> Let $\mathrm{T}_x\mathrm{M}$ be the set of all derivations at $x$ such that $\mathbf{v}_x \in \mathrm{T}_x\mathrm{M}$. With $\mathrm{T}_x\mathrm{M}$ denoted as the **tangent space** at $x$. 
-
-We may think of the tangent space at a point $x \in \mathrm{M}$ as a space attached to $x$ on the differential manifold $M$. 
-
-> *Theorem 1*: let $M$ be a differential manifold and let $x \in \mathrm{M}$, the tangent space $\mathrm{T}_x\mathrm{M}$ is a vector space. 
-
-??? note "*Proof*:"
-
-    Will be added later.
-
-Thus, the tangent space is a vector space attached to $x \in \mathrm{M}$ on the differential manifold. It follows that its vectors have interesting properties. 
-
-> *Theorem 2*: let $M$ be a differential manifold, let $x \in \mathrm{M}$ and let $\mathbf{v}_x \in \mathrm{T}_x\mathrm{M}$, then we have that
->
-> $$
->   \forall f \in C^{\infty}(\mathrm{M}): \mathbf{v}_x f = v^i \partial_i f(x),
-> $$
->
-> such that $\mathbf{v}_x = v^i \partial_i \in \mathrm{T}_x\mathrm{M}$ is denoted as a **tangent vector** in the tangent space $\mathrm{T}_x\mathrm{M}$. 
-
-
-??? note "*Proof*:"
-
-    Will be added later.
-
-Theorem 2 adds the notion of tangent vectors to the explanation of the tangent space. The tangent space at a point on the manifold thus represents the space of tangent vectors.
-
-> *Proposition 1*: let $M$ be a differential manifold of $\dim\mathrm{M} = n \in \mathbb{N}$. The tangent space $\mathrm{T}_x\mathrm{M}$ has dimension $n$ such that
-> 
-> $$
->   \forall x \in \mathrm{M}: \dim \mathrm{T}_x\mathrm{M} = \dim\mathrm{M}
-> $$
-> 
-> and is span by the vector basis $\{\partial_i\}_{i=1}^n$. 
-
-??? note "*Proof*:"
-
-    Will be added later.
-
-Proposition 1 states that the tangent space is of the same dimension as the manifold and its basis are partial derivative operators. In the context of the [covariant basis](), this definition of the basis leaves out the coordinate map, but is in fact equivalent to the covariant basis.
-
-As a last step in the explanation, we may think of the 2 dimensional surface of a sphere, which may define a differential manifold $M$. The tangent space at a point $x \in \mathrm{M}$ on the surface of the sphere may then be compared to the tangent plane to the sphere attached at point $x \in \mathrm{M}$. The catch is that the 3 dimensional space necessary to understand this construction exists only in our imagination and not in the mathematical construct.
-
-> *Definition 7*: let $M$ be a differential manifold, the collection of tangent spaces $\mathrm{T}_x\mathrm{M}$ for all $x \in \mathrm{M}$ define the **tangent bundle** as 
->
-> $$
->   \mathrm{TM} = \bigcup_{x \in \mathrm{M}} \mathrm{T}_x\mathrm{M}.
-> $$
-
-In particular, we may think of the tangent bundle $\mathrm{TM}$ as a subspace $\mathrm{TM} \subset V$ of the vector bundle $V$ for a differential manifold. With the special properties given in theorem 2 and proposition 1. 
-
-The connection of each tangent vector to its base point may be formalised with the projection map $\pi$ which in this case is given by
-
-$$
-    \pi: \mathrm{TM} \to\mathrm{M}: (x, \mathbf{v}) \mapsto \pi(x, \mathbf{v}) \overset{\text{def}}{=} x,
-$$
-
-and its inverse
-
-$$
-    \pi^{-1}:\mathrm{M} \to \mathrm{TM}: x \mapsto \pi^{-1}(x) \overset{\text{def}}{=} \mathrm{T}_x\mathrm{M}.
-$$
-
-> *Definition 8*: a vector field $\mathbf{v}$ on a differential manifold $M$ is a section
->
-> $$
->   \mathbf{v} \in \Gamma(\mathrm{TM}),
-> $$
->
-> of the tangent bundle $\mathrm{TM}$. 
-
-## Cotangent bundles
-
-> *Definition 9*: let $M$ be a differential manifold and $\mathrm{T}_x\mathrm{M}$ the tangent space at $x \in \mathrm{M}$. We define the **cotangent space** $\mathrm{T}_x^*\mathrm{M}$ as the dual space of $\mathrm{T}_x\mathrm{M}$
->
-> $$
->   \mathrm{T}_x^*\mathrm{M} = (\mathrm{T}_x\mathrm{M})^*.
-> $$
-> 
-> Then every element $\bm{\omega}_x \in \mathrm{T}_x^*\mathrm{M}$ is a linear map $\bm{\omega}_x: \mathrm{T}_x\mathrm{M} \to \mathbb{K}$ denoted as the **cotangent vector**. 
-
-This definition is a logical consequence of the notion of the [dual vector space](). It then also follows that the dual cotangent space is isomorphic to the tangent space at a point $x \in \mathrm{M}$. 
-
-> *Theorem 3*: let $\mathrm{M}$ be a differential manifold of $\dim \mathrm{M} = n \in \mathbb{N}$, then we have that for every $x \in \mathrm{M}$ the basis $\{dx^i\}_{i=1}^n$ of $\mathrm{T}_x^*\mathrm{M}$ is uniquely determined by
->
-> $$
->   dx^i(\partial_j) = \delta^i_j,
-> $$
->
-> for each basis $\{\partial_j\}_{j=1}^n$ in $\mathrm{T}_x\mathrm{M}$. 
-
-??? note "*Proof*:"
-
-    The proof follows directly from theorem 1 in [dual vector spaces](). 
-
-The choice of $dx^i$ can be explained by taking the differential $df = \partial_i f dx^i \in \mathrm{T}_x^*\mathrm{M}$ with $f \in C^\infty(\mathrm{M})$. Then if we take
-
-$$
-    \mathbf{k}_x(df, \mathbf{v}) = \mathbf{k}(\partial_i f dx^i, v^j \partial_j) = v^j \partial_i f \mathbf{k}(dx^i, \partial_j) = v^j \partial_i f \delta^i_j = v^i \partial_i f = \mathbf{v} f,
-$$
-
-with $\mathbf{k}_x: \mathrm{T}_x^*\mathrm{M} \times \mathrm{T}_x\mathrm{M} \to \mathbb{K}$ the Kronecker tensor at $x \in \mathrm{M}$. Which shows that defining the basis of the cotangent space as differentials corresponds with respect to the basis of the tangent space.
-
-So, a cotangent vector $\bm{\omega}_x \in \mathrm{T}_x^*\mathrm{M}$ may be decomposed into
-
-$$
-    \bm{\omega}_x = \omega_i dx^i. 
-$$
-
-## Push forward and pull back
-
-> *Definition 10*: let $\mathrm{M}, \mathrm{N}$ be two differential manifolds with $\dim \mathrm{N} \geq \dim \mathrm{M}$ and let $\psi: \mathrm{M} \to \mathrm{N}$ be the diffeomorphism between the manifolds. Then we define the **pull back** $\psi^*$ and **push forward** $\psi_*$ operators, such that for $\mathbf{v} \in \mathrm{T}_x \mathrm{M}$ and $\bm{\omega} \in \mathrm{T}_{\psi(x)}^* \mathrm{M}$ we have
->
-> $$
->   \mathbf{k}_x(\psi^* \bm{\omega}, \mathbf{v}) = \mathbf{k}_{\psi(x)}(\bm{\omega}, \psi_* \mathbf{v}),
-> $$
->
-> for all $x \in \mathrm{M}$.
-
-Which indicates the proper separation between the elements of both spaces.

docs/en/mathematics/differential-geometry/linear-connections.md

@@ -0,0 +1,18 @@
+# Linear connections
+
+Let $\mathrm{M}$ be a differential manifold with $\dim \mathrm{M} = n \in \mathbb{N}$ used throughout the section. Let $\mathrm{TM}$ and $\mathrm{T^*M}$ denote the tangent and cotangent bundle. 
+
+> *Definition 1*: a **linear connection** on the fiber bundle $\mathscr{B}$ is a map
+>
+> $$
+>   \nabla: \Gamma(\mathrm{TM}) \times \Gamma(\mathscr{B}) \to \Gamma(\mathscr{B}): (\mathbf{v}, \mathbf{T}) \mapsto \nabla_\mathbf{v} \mathbf{T}, 
+> $$
+>
+> satisfying the following properties, if $f,g \in C^\infty(\mathrm{M})$, $\mathbf{v} \in \mathrm{TM}$ and $\mathbf{T}, \mathbf{S} \in \mathscr{B}$ then
+>
+> 1. $\nabla_{f\mathbf{v}} \mathbf{T} = f \nabla_\mathbf{v} \mathbf{T}$
+> 2. $\nabla_\mathbf{v} (f \mathbf{T} + g \mathbf{S}) = (\nabla_\mathbf{v} f) \mathbf{T} + f \nabla_\mathbf{v} \mathbf{T} + (\nabla_\mathbf{v} g) \mathbf{S} + g \nabla_{\mathbf{v}} \mathbf{S}$, 
+> 3. $\nabla_\mathbf{v} f = \mathbf{v} f = \mathbf{k}(df, \mathbf{v})$. 
+
+From property 3 it becomes clear that $\nabla_\mathbf{v}$ is an analogue of a directional derivative. 
+

docs/en/mathematics/differential-geometry/tangent-spaces.md

@@ -0,0 +1,136 @@
+# Tangent spaces
+
+Let $\mathrm{M}$ be a differential manifold with $\dim \mathrm{M} = n \in \mathbb{N}$ used throughout the section.
+
+## Definition
+
+> *Definition 1*: let $f \in C^{\infty}(\mathrm{M})$ with $C^{\infty}$ the class of [smooth functions]() and $M$ a differential manifold. A derivation of $f$ at $x \in \mathrm{M}$ is defined as a linear map $\mathbf{v}_x: C^\infty(\mathrm{M}) \to \mathbb{K}$ that satisfies
+>
+> $$
+>   \forall f,g \in C^{\infty}(\mathrm{M}): \mathbf{v}_x(f g) = (\mathbf{v}_xf) g + f (\mathbf{v}_x g).
+> $$
+>
+> Let $\mathrm{T}_x\mathrm{M}$ be the set of all derivations at $x$ such that $\mathbf{v}_x \in \mathrm{T}_x\mathrm{M}$. With $\mathrm{T}_x\mathrm{M}$ denoted as the **tangent space** at $x$. 
+
+We may think of the tangent space at a point $x \in \mathrm{M}$ as a space attached to $x$ on the differential manifold $M$. 
+
+## Properties of tangent spaces
+
+> *Theorem 1*: let $M$ be a differential manifold and let $x \in \mathrm{M}$, the tangent space $\mathrm{T}_x\mathrm{M}$ is a vector space. 
+
+??? note "*Proof*:"
+
+    Will be added later.
+
+Thus, the tangent space is a vector space attached to $x \in \mathrm{M}$ on the differential manifold. It follows that its vectors have interesting properties. 
+
+> *Theorem 2*: let $M$ be a differential manifold, let $x \in \mathrm{M}$ and let $\mathbf{v}_x \in \mathrm{T}_x\mathrm{M}$, then we have that
+>
+> $$
+>   \forall f \in C^{\infty}(\mathrm{M}): \mathbf{v}_x f = v^i \partial_i f(x),
+> $$
+>
+> such that $\mathbf{v}_x = v^i \partial_i \in \mathrm{T}_x\mathrm{M}$ is denoted as a **tangent vector** in the tangent space $\mathrm{T}_x\mathrm{M}$. 
+
+
+??? note "*Proof*:"
+
+    Will be added later.
+
+Theorem 2 adds the notion of tangent vectors to the explanation of the tangent space. The tangent space at a point on the manifold thus represents the space of tangent vectors.
+
+> *Proposition 1*: let $M$ be a differential manifold of $\dim\mathrm{M} = n \in \mathbb{N}$. The tangent space $\mathrm{T}_x\mathrm{M}$ has dimension $n$ such that
+> 
+> $$
+>   \forall x \in \mathrm{M}: \dim \mathrm{T}_x\mathrm{M} = \dim\mathrm{M}
+> $$
+> 
+> and is span by the vector basis $\{\partial_i\}_{i=1}^n$. 
+
+??? note "*Proof*:"
+
+    Will be added later.
+
+Proposition 1 states that the tangent space is of the same dimension as the manifold and its basis are partial derivative operators. In the context of the [covariant basis](), this definition of the basis leaves out the coordinate map, but is in fact equivalent to the covariant basis.
+
+As a last step in the explanation, we may think of the 2 dimensional surface of a sphere, which may define a differential manifold $M$. The tangent space at a point $x \in \mathrm{M}$ on the surface of the sphere may then be compared to the tangent plane to the sphere attached at point $x \in \mathrm{M}$. The catch is that the 3 dimensional space necessary to understand this construction exists only in our imagination and not in the mathematical construct.
+
+## Tangent bundle
+
+> *Definition 2*: let $M$ be a differential manifold, the collection of tangent spaces $\mathrm{T}_x\mathrm{M}$ for all $x \in \mathrm{M}$ define the **tangent bundle** as 
+>
+> $$
+>   \mathrm{TM} = \bigcup_{x \in \mathrm{M}} \mathrm{T}_x\mathrm{M}.
+> $$
+
+In particular, we may think of the tangent bundle $\mathrm{TM}$ as a subspace $\mathrm{TM} \subset V$ of the fiber bundle $V$ for a differential manifold. With the special properties given in theorem 2 and proposition 1. 
+
+The connection of each tangent vector to its base point may be formalised with the projection map $\pi$ which in this case is given by
+
+$$
+    \pi: \mathrm{TM} \to\mathrm{M}: (x, \mathbf{v}) \mapsto \pi(x, \mathbf{v}) \overset{\text{def}}{=} x,
+$$
+
+and its inverse
+
+$$
+    \pi^{-1}:\mathrm{M} \to \mathrm{TM}: x \mapsto \pi^{-1}(x) \overset{\text{def}}{=} \mathrm{T}_x\mathrm{M}.
+$$
+
+> *Definition 3*: a vector field $\mathbf{v}$ on a differential manifold $M$ is a section
+>
+> $$
+>   \mathbf{v} \in \Gamma(\mathrm{TM}),
+> $$
+>
+> of the tangent bundle $\mathrm{TM}$. 
+
+## Cotangent spaces
+
+> *Definition 4*: let $M$ be a differential manifold and $\mathrm{T}_x\mathrm{M}$ the tangent space at $x \in \mathrm{M}$. We define the **cotangent space** $\mathrm{T}_x^*\mathrm{M}$ as the dual space of $\mathrm{T}_x\mathrm{M}$
+>
+> $$
+>   \mathrm{T}_x^*\mathrm{M} = (\mathrm{T}_x\mathrm{M})^*.
+> $$
+> 
+> Then every element $\bm{\omega}_x \in \mathrm{T}_x^*\mathrm{M}$ is a linear map $\bm{\omega}_x: \mathrm{T}_x\mathrm{M} \to \mathbb{K}$ denoted as the **cotangent vector**. 
+
+This definition is a logical consequence of the notion of the [dual vector space](). It then also follows that the dual cotangent space is isomorphic to the tangent space at a point $x \in \mathrm{M}$. 
+
+> *Theorem 3*: let $\mathrm{M}$ be a differential manifold of $\dim \mathrm{M} = n \in \mathbb{N}$, then we have that for every $x \in \mathrm{M}$ the basis $\{dx^i\}_{i=1}^n$ of $\mathrm{T}_x^*\mathrm{M}$ is uniquely determined by
+>
+> $$
+>   dx^i(\partial_j) = \delta^i_j,
+> $$
+>
+> for each basis $\{\partial_j\}_{j=1}^n$ in $\mathrm{T}_x\mathrm{M}$. 
+
+??? note "*Proof*:"
+
+    The proof follows directly from theorem 1 in [dual vector spaces](). 
+
+The choice of $dx^i$ can be explained by taking the differential $df = \partial_i f dx^i \in \mathrm{T}_x^*\mathrm{M}$ with $f \in C^\infty(\mathrm{M})$. Then if we take
+
+$$
+    \mathbf{k}_x(df, \mathbf{v}) = \mathbf{k}(\partial_i f dx^i, v^j \partial_j) = v^j \partial_i f \mathbf{k}(dx^i, \partial_j) = v^j \partial_i f \delta^i_j = v^i \partial_i f = \mathbf{v} f,
+$$
+
+with $\mathbf{k}_x: \mathrm{T}_x^*\mathrm{M} \times \mathrm{T}_x\mathrm{M} \to \mathbb{K}$ the Kronecker tensor at $x \in \mathrm{M}$. Which shows that defining the basis of the cotangent space as differentials corresponds with respect to the basis of the tangent space.
+
+So, a cotangent vector $\bm{\omega}_x \in \mathrm{T}_x^*\mathrm{M}$ may be decomposed into
+
+$$
+    \bm{\omega}_x = \omega_i dx^i. 
+$$
+
+In the context of the [contravariant basis](), this definition of the basis leaves out the coordinate map, but is in fact equivalent to the contravariant basis.
+
+## Cotangent bundle
+
+> *Definition 5*: let $M$ be a differential manifold, the collection of cotangent spaces $\mathrm{T}_x^*\mathrm{M}$ for all $x \in \mathrm{M}$ define the **cotangent bundle** as 
+>
+> $$
+>   \mathrm{T^*M} = \bigcup_{x \in \mathrm{M}} \mathrm{T}_x^*\mathrm{M}.
+> $$
+
+Thus, we may think of the cotangent bundle $\mathrm{T^*M}$ as a subspace $\mathrm{T^*M} \subset V^*$ of the dual fiber bundle $V^*$ for a differential manifold.

docs/en/mathematics/differential-geometry/transformations.md

@@ -0,0 +1,35 @@
+# Transformations
+
+Let $\mathrm{M}$ be a differential manifold with $\dim \mathrm{M} = n \in \mathbb{N}$ used throughout the section. Let $\mathrm{TM}$ and $\mathrm{T^*M}$ denote the tangent and cotangent bundle. 
+
+## Push forward and pull back
+
+> *Definition 1*: let $\mathrm{M}, \mathrm{N}$ be two differential manifolds with $\dim \mathrm{N} \geq \dim \mathrm{M}$ and let $\psi: \mathrm{M} \to \mathrm{N}$ be the diffeomorphism between the manifolds. Then we define the **pull back** $\psi^*$ and **push forward** $\psi_*$ operators, such that for $\mathbf{v} \in \mathrm{T}_x \mathrm{M}$ and $\bm{\omega} \in \mathrm{T}_{\psi(x)}^* \mathrm{M}$ we have
+>
+> $$
+>   \mathbf{k}_x(\psi^* \bm{\omega}, \mathbf{v}) = \mathbf{k}_{\psi(x)}(\bm{\omega}, \psi_* \mathbf{v}),
+> $$
+>
+> for all $x \in \mathrm{M}$.
+
+Which indicates the proper separation between the elements of both spaces.
+
+## Basis transformation
+
+Let $\psi: \mathscr{D}(\mathrm{M}) \to \mathrm{M}: x \mapsto \psi(x) \overset{\text{def}}{=} \overline{x}$ be an active coordinate transformation from a point $x$ to a point $\overline{x}$ on $\mathrm{M}$. Then we have a basis $\{\partial_i\}_{i=1}^n \subset \mathrm{T}_x\mathrm{M}$ for the tangent space $\mathrm{T}_x\mathrm{M}$ at $x$ and a basis $\{\overline{\partial_i}\}_{i=1}^n \subset \mathrm{T}_{\overline{x}}\mathrm{M}$ for the tangent space $\mathrm{T}_{\overline{x}}\mathrm{M}$ at $\overline{x}$. Which are related by
+
+$$
+    \partial_i = J^j_i \overline{\partial_j} = \partial_i \psi^j(x) \overline{\partial_j}, 
+$$
+
+with $J^j_i = \partial_i \psi^j(x)$ the [Jacobian]() at $x \in \mathrm{M}$. For it to make sense, it helps to change notation to 
+
+$$
+    \frac{\partial}{\partial x_i} = \frac{\partial \overline{x}^j}{\partial x_i} \frac{\partial}{\partial \overline{x}_j} = \frac{\partial \psi^j}{\partial x_i} \frac{\partial}{\partial \overline{x}_j}. 
+$$
+
+Similarly, we have a basis $\{dx^i\}_{i=1}^n \subset \mathrm{T}_x^*\mathrm{M}$ for the cotangent space $\mathrm{T}_x\mathrm{M}$ at $x$ and a basis $\{d\overline{x}^i\}_{i=1}^n \subset \mathrm{T}_{\overline{x}}^*\mathrm{M}$ for the cotangent space $\mathrm{T}_{\overline{x}}^*\mathrm{M}$ at $\overline{x}$. Which are related by
+
+$$
+    d\overline{x}^i = J^i_j dx^j = \partial_j \psi^i(x) dx^j. 
+$$

docs/en/mathematics/topology/fiber-bundles.md

@@ -0,0 +1,57 @@
+# Fiber bundles
+
+Let $X$ be a manifold over a field $F$. 
+
+> *Definition 1*: a **fiber** $V_x$ at a point $x \in X$ on a manifold is a finite dimensional vector space. With the collection of fibers $V_x$ for all $x \in X$ define the **fiber bundle** as
+>
+> $$
+>   V = \bigcup_{x \in X} V_x.
+> $$
+
+Then by definition we have the projection map $\pi$ given by
+
+$$
+    \pi: V \to X: (x,\mathbf{v}) \mapsto \pi(x, \mathbf{v}) \overset{\text{def}}{=} x,
+$$
+
+and its inverse
+
+$$
+    \pi^{-1}: X \to V: x \mapsto \pi(x) \overset{\text{def}}{=} V_x.
+$$
+
+Similarly, a dual fiber $V_x^*$ may be defined for $x \in X$, with its fiber bundle defined by
+
+$$
+    V^* = \bigcup_{x \in X} V_x^*.
+$$
+
+> *Definition 2*: a **tensor fiber** $\mathscr{B}_x$ at a point $x \in X$ on a manifold is defined as
+>
+> $$
+>   \mathscr{B}_x = \bigcup_{p,q \in \mathbb{N}} \mathscr{T}^p_q(V_x).
+> $$
+>
+> With the collection of tensor fibers $\mathscr{B}_x$ for all $x \in X$ define the **tensor fiber bundle** as
+>
+> $$
+>   \mathscr{B} = \bigcup_{x \in X} \mathscr{B}_x.
+> $$
+
+Then for a point $x \in X$ we have a tensor $\mathbf{T} \in \mathscr{B}_x$ such that
+
+$$
+    \mathbf{T} = T^{ij}_k \mathbf{e}_i \otimes \mathbf{e}_j \otimes \mathbf{\hat e}^k, 
+$$
+
+with $T^{ij}_k \in \mathbb{K}$ holors of $\mathbf{T}$. Furthermore, we have a basis $\{\mathbf{e}_i\}_{i=1}^n$ of $V_x$ and a basis $\{\mathbf{\hat e}^i\}_{i=1}^n$ of $V_x^*$. 
+
+> *Definition 3*: a tensor field $\mathbf{T}$ on a manifold $X$ is a [section]()
+>
+> $$
+>   \mathbf{T} \in \Gamma(X, \mathscr{B}),
+> $$
+>
+> of the tensor fiber bundle $\mathscr{B}$. 
+
+Therefore, a tensor field assigns a tensor fiber (or tensor) to each point on a section of the  manifold. These tensors may vary smoothly along the section of the manifold.