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# Differential manifolds # Differential manifolds
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 $\mathbb{K} = \mathbb{C}.$ In the following sections of differential geometry we make use of the Einstein summation convention and $\mathbb{K} = \mathbb{R}$ or $\mathbb{K} = \mathbb{C}$.
## Definition ## Definition
@ -38,4 +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 $M$. 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.

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# Dual vector spaces # Dual vector spaces
We have a $n \in \mathbb{N}$ finite dimensional vector space $V$ such that $\dim V = n$, with a basis $\{\mathbf{e}_i\}_{i=1}^n.$ 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 $\mathbb{K} = \mathbb{R} \lor\mathbb{K} = \mathbb{C}$. We have a $n \in \mathbb{N}$ finite dimensional vector space $V$ such that $\dim V = n$, with a basis $\{\mathbf{e}_i\}_{i=1}^n.$ In the following sections we make use of the Einstein summation convention introduced and $\mathbb{K} = \mathbb{R} \lor\mathbb{K} = \mathbb{C}$.
> *Definition 1*: let $\mathbf{\hat f}: V \to \mathbb{K}$ be a **covector** or **linear functional** on $V$ if for all $\mathbf{v}_{1,2} \in V$ and $\lambda, \mu \in \mathbb{K}$ we have > *Definition 1*: let $\mathbf{\hat f}: V \to \mathbb{K}$ be a **covector** or **linear functional** on $V$ if for all $\mathbf{v}_{1,2} \in V$ and $\lambda, \mu \in \mathbb{K}$ we have
> >
@ -54,4 +54,4 @@ From theorem 1 it follows that for each covector basis $\{\mathbf{\hat e}^i\}$ o
??? note "*Proof*:" ??? note "*Proof*:"
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A \mathbf{x} - \lambda \mathbf{x} = (A - \lambda I) \mathbf{x} = \mathbf{0}, A \mathbf{x} - \lambda \mathbf{x} = (A - \lambda I) \mathbf{x} = \mathbf{0},
$$ $$
which implies that $(A - \lambda I)$ is singular and $\det(A - \lambda I) = 0$ by [definition](../determinants/#properties-of-determinants). which implies that $(A - \lambda I)$ is singular and $\det(A - \lambda I) = 0$ by [definition](determinants.md#properties-of-determinants).
The eigenvalues $\lambda$ may thus be determined from the **characteristic polynomial** of degree $n$ that is obtained from $\det (A - \lambda I) = 0$. In particular, the eigenvalues are the roots of this polynomial. The eigenvalues $\lambda$ may thus be determined from the **characteristic polynomial** of degree $n$ that is obtained from $\det (A - \lambda I) = 0$. In particular, the eigenvalues are the roots of this polynomial.
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??? note "*Proof*:" ??? note "*Proof*:"
Let $A$ be a $n \times n$ matrix and let $\lambda_1, \dots, \lambda_n \in \mathbb{K}$ be the eigenvalues of $A$. It follows from the [fundamental theorem of algebra](../../number-theory/complex-numbers/#roots-of-polynomials) that Let $A$ be a $n \times n$ matrix and let $\lambda_1, \dots, \lambda_n \in \mathbb{K}$ be the eigenvalues of $A$. It follows from the [fundamental theorem of algebra](../number-theory/complex-numbers.md#roots-of-polynomials) that
$$ $$
\det (A - \lambda I) = (\lambda_1 - \lambda)(\lambda_2 - \lambda) \cdots (\lambda_n - \lambda), \det (A - \lambda I) = (\lambda_1 - \lambda)(\lambda_2 - \lambda) \cdots (\lambda_n - \lambda),
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### Hermitian case ### Hermitian case
The following section is for the special case that a matrix is [Hermitian](../matrices/matrix-arithmatic/#hermitian-matrix). The following section is for the special case that a matrix is [Hermitian](matrices/matrix-arithmetic.md#hermitian-matrix).
> *Theorem 7*: the eigenvalues of a Hermitian matrix are real. > *Theorem 7*: the eigenvalues of a Hermitian matrix are real.
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??? note "*Proof*:" ??? note "*Proof*:"
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### Fundamental subspaces ### Fundamental subspaces
Let $V$ be an Euclidean inner product space $V = \mathbb{R}^n$ with its inner product defined by the [scalar product](../inner-product-spaces/#euclidean-inner-product-spaces). With this definition of the inner product on $V$ the following theorem may be posed. Let $V$ be an Euclidean inner product space $V = \mathbb{R}^n$ with its inner product defined by the [scalar product](inner-product-spaces.md#euclidean-inner-product-spaces). With this definition of the inner product on $V$ the following theorem may be posed.
> *Theorem 1*: let $A$ be an $m \times n$ matrix, then > *Theorem 1*: let $A$ be an $m \times n$ matrix, then
> >
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S^\perp = R(X^T)^\perp = N(X), S^\perp = R(X^T)^\perp = N(X),
$$ $$
from the [rank nullity theorem](../vector-spaces/#rank-and-nullity) it follows that from the [rank nullity theorem](vector-spaces.md#rank-and-nullity) it follows that
$$ $$
\dim S^\perp = \dim N(X) = n - r. \dim S^\perp = \dim N(X) = n - r.
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\mathbf{\hat x} = (A^T A)^{-1} A^T \mathbf{b}, \mathbf{\hat x} = (A^T A)^{-1} A^T \mathbf{b},
$$ $$
is the unique solution of the normal equations for $A$ nonsingular and consequently, the unique least squares solution of the system $A \mathbf{x} = \mathbf{b}$. is the unique solution of the normal equations for $A$ nonsingular and consequently, the unique least squares solution of the system $A \mathbf{x} = \mathbf{b}$.

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# Tensor formalism # Tensor formalism
We have a $n \in \mathbb{N}$ finite dimensional vector space $V$ such that $\dim V = n$, with a basis $\{\mathbf{e}_i\}_{i=1}^n$ and a corresponding dual space $V^*$ with a basis $\{\mathbf{\hat e}^i\}.$ 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 $\mathbb{K} = \mathbb{R} \lor\mathbb{K} = \mathbb{C}.$ We have a $n \in \mathbb{N}$ finite dimensional vector space $V$ such that $\dim V = n$, with a basis $\{\mathbf{e}_i\}_{i=1}^n$ and a corresponding dual space $V^*$ with a basis $\{\mathbf{\hat e}^i\}.$ In the following sections we make use of the Einstein summation convention and $\mathbb{K} = \mathbb{R} \lor\mathbb{K} = \mathbb{C}$.
## Definition ## Definition
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> \mathbf{g}(\mathbf{e}^i) = g_{ij} \mathbf{\hat e}^j. > \mathbf{g}(\mathbf{e}^i) = g_{ij} \mathbf{\hat e}^j.
> $$ > $$
So far, a vector space $V$ and its associated dual space $V^*$ have been introduced as a priori independent entities. An inner product provides us with an explicit mechanism to construct a bijective linear mapping associated with each vector by virtue of the metric. So far, a vector space $V$ and its associated dual space $V^*$ have been introduced as a priori independent entities. An inner product provides us with an explicit mechanism to construct a bijective linear mapping associated with each vector by virtue of the metric.

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It may be observed that the Hamiltonian $\mathcal{H}$ and [generalised energy](/en/physics/mechanics/lagrangian-mechanics/lagrange-generalizations/#the-generalized-energy) $h$ are identical. Note however that $\mathcal{H}$ must be expressed in $(\mathbf{q},\mathbf{p},t)$ which is not the case for $h$. It may be observed that the Hamiltonian $\mathcal{H}$ and [generalised energy](../lagrangian-mechanics/lagrange-generalizations.md#the-generalized-energy) $h$ are identical. Note however that $\mathcal{H}$ must be expressed in $(\mathbf{q},\mathbf{p},t)$ which is not the case for $h$.
> *Proposition 2*: a coordinate $q_j$ is cyclic if > *Proposition 2*: a coordinate $q_j$ is cyclic if
> >

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# Hamiltonian formalism of mechanics # Hamiltonian formalism of mechanics
The Hamiltonian formalism of mechanics is based on the definitions posed by [Lagrangian mechanics](/en/physics/mechanics/lagrangian-mechanics/lagrangian-formalism) and the axioms, postulates and principles posed in the [Newtonian formalism](/en/physics/mechanics/newtonian-mechanics/newtonian-formalism/). The Hamiltonian formalism of mechanics is based on the definitions posed by [Lagrangian mechanics](../lagrangian-mechanics/lagrangian-formalism.md) and the axioms, postulates and principles posed in the [Newtonian formalism](../newtonian-mechanics/newtonian-formalism.md).
Where the Lagrangian formalism used the [principle of virtual work](/en/physics/mechanics/lagrangian-mechanics/lagrange-equations/#principle-of-virtual-work) to derive the Lagrangian equations of motion, the Hamiltonian formalism will derive the Lagrangian equations with the stationary action principle. A derivative of Fermat's principle of least time. Where the Lagrangian formalism used the [principle of virtual work](../lagrangian-mechanics/lagrange-equations.md#principle-of-virtual-work) to derive the Lagrangian equations of motion, the Hamiltonian formalism will derive the Lagrangian equations with the stationary action principle. A derivative of Fermat's principle of least time.
In Hamilton's formulation the stationary action principle is referred to as Hamilton's principle. In Hamilton's formulation the stationary action principle is referred to as Hamilton's principle.

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# Lagrangian formalism of mechanics # Lagrangian formalism of mechanics
The Lagrangian formalism of mechanics is based on the axioms, postulates and principles posed in the [Newtonian formalism](/en/physics/mechanics/newtonian-mechanics/newtonian-formalism/). The Lagrangian formalism of mechanics is based on the axioms, postulates and principles posed in the [Newtonian formalism](../newtonian-mechanics/newtonian-formalism.md).
## Configuration of a system ## Configuration of a system
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## Potential energy ## Potential energy
> *Definition 1*: a force field $\mathbf{F}$ is conservative if it is [irrotational](../../mathematical-physics/vector-analysis/vector-operators/#potentials) > *Definition 1*: a force field $\mathbf{F}$ is conservative if it is [irrotational](../../mathematical-physics/vector-analysis/vector-operators.md#potentials)
> >
> $$ > $$
> \nabla \times \mathbf{F} = 0, > \nabla \times \mathbf{F} = 0,