Difference between revisions of "Time scale"

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A ''time scale'' is a set $\mathbb{T} \subset \mathbb{R}$ which is closed under the standard topology of $\mathbb{R}$. Sometimes we deal with the set $\mathbb{T}^{\kappa} = \mathbb{T} \setminus \left\{ \sup \mathbb{T} \right\}$ (if $\sup \mathbb{T}=\infty$ then $\mathbb{T}^{\kappa}=\mathbb{T}$). Given a time scale we define the ''jump operator'' $\sigma \colon \mathbb{T} \rightarrow \mathbb{T}$ by the formula
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A time scale is a set $\mathbb{T} \subset \mathbb{R}$ which is closed under the standard topology of $\mathbb{R}$. Sometimes we deal with the set $\mathbb{T}^{\kappa} = \mathbb{T} \setminus \left\{ \sup \mathbb{T} \right\}$ (if $\sup \mathbb{T}=\infty$ then $\mathbb{T}^{\kappa}=\mathbb{T}$). For some set $X$, let $f \colon \mathbb{T} \rightarrow X$. The following is a common notation using the [[forward jump]] operator: $f^{\sigma} \colon  \mathbb{T}^{\kappa} \rightarrow X$ is given by the formula $f^{\sigma}(t)=f(\sigma(t))$. Similarly the [[backward jump]] is used to define the function $f^{\rho}$.
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$$\sigma(t) := \inf \left\{ x \in \mathbb{T} \colon x > t \right\}.$$
 
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Let $f \colon \mathbb{T} \rightarrow \mathbb{R}$. The following is a common notation: $f^{\sigma} \colon  \mathbb{T}^{\kappa} \rightarrow \mathbb{R}$ is given by the formula $f^{\sigma}(t)=f(\sigma(t))$. A similar operator, the backward jump operator $\rho \colon \mathbb{T}\rightarrow \mathbb{T}$ is defined by the formula
 
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$$\rho(t) = \sup \{ x \in \mathbb{T} \colon x<t\}.$$
 
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Let $t \in \mathbb{T}$. We say that $t$ is right-scattered if $\sigma(t)>t$ (left-scattered if $\rho(t)<t$) and that $t$ is right-dense if $\sigma(t)=t$ (left-dense if $\rho(t)=t$).
 
  
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The ''graininess operator'' is the function $\mu \colon \mathbb{T} \rightarrow \mathbb{R}^+ \cup \{0\}$ is defined by the formula
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To every time scale we have "standard" calculus operators: the [[delta_derivative | $\Delta$-derivative]] and [[delta_integration | $\Delta$-integral]], however there are also different types of other derivatives and integrals such as the [[nabla derivative | $\nabla$-derivative]] and the [[nabla integral | $\nabla$-integral]].
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$$\mu(t) := \sigma(t)-t.$$
 
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The ''backwards graininess operator'' is the function $\nu \colon \mathbb{T} \rightarrow \mathbb{R}^+ \cup \{0\}$ is defined by the formula
 
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$$\nu(t) := t - \rho(t).$$
 
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To every time scale we have a standard calculus operators: the [[delta_derivative | $\Delta$-derivative]] and [[delta_integration | $\Delta$-integral]], however there are also different types of derivatives and integrals such as the [[nabla derivative | $\nabla$-derivative]] and the [[nabla integral | $\nabla$-integral]].  
 
  
 
=The set of time scales=
 
=The set of time scales=

Revision as of 22:47, 23 February 2016

A time scale is a set $\mathbb{T} \subset \mathbb{R}$ which is closed under the standard topology of $\mathbb{R}$. Sometimes we deal with the set $\mathbb{T}^{\kappa} = \mathbb{T} \setminus \left\{ \sup \mathbb{T} \right\}$ (if $\sup \mathbb{T}=\infty$ then $\mathbb{T}^{\kappa}=\mathbb{T}$). For some set $X$, let $f \colon \mathbb{T} \rightarrow X$. The following is a common notation using the forward jump operator: $f^{\sigma} \colon \mathbb{T}^{\kappa} \rightarrow X$ is given by the formula $f^{\sigma}(t)=f(\sigma(t))$. Similarly the backward jump is used to define the function $f^{\rho}$.

To every time scale we have "standard" calculus operators: the $\Delta$-derivative and $\Delta$-integral, however there are also different types of other derivatives and integrals such as the $\nabla$-derivative and the $\nabla$-integral.

The set of time scales

Let $\mathcal{H} = \{\mathbb{T} \subset \mathbb{R} \colon \mathbb{T}$ is a closed set $\}$. A set like this can be given a standard topological structure making it the hyperspace $\mathcal{H}=\mathrm{CL}(\mathbb{R})$. We can characterize time scales using the Cantor-Bendixson derivative -- a time scale $\mathbb{T}$ is the union of a perfect set and a countable set.

Examples of time scales

  1. The real line: $\mathbb{R}$
  2. The integers: $\mathbb{Z} = \{\ldots, -1,0,1,\ldots\}$
  3. Multiples of integers: $h\mathbb{Z} = \{ht \colon t \in \mathbb{Z}\}$
  4. Quantum numbers ($q>1$): $\overline{q^{\mathbb{Z}}}$
  5. Quantum numbers ($q<1$): $\overline{q^{\mathbb{Z}}}$
  6. Square integers: $\mathbb{Z}^2 = \{t^2 \colon t \in \mathbb{Z} \}$
  7. Harmonic numbers: $\mathbb{H}=\left\{\displaystyle\sum_{k=1}^n \dfrac{1}{k} \colon n \in \mathbb{Z}^+ \right\}$
  8. The closure of the unit fractions: $\overline{\left\{\dfrac{1}{n} \colon n \in \mathbb{Z}^+\right\}}$
  9. Isolated points: $\mathbb{T}=\{\ldots, t_{-1}, t_{0}, t_1, \ldots\}$

Applications of time scales

  1. Control theory, see this and this and this
  2. Economics, see this and this
  3. Ecology, see this
  4. Possible application to geophysics here
  5. Biological systems here
  6. Population model for flies here
  7. this
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