Difference between revisions of "Time scale"

From timescalewiki
Jump to: navigation, search
Line 25: Line 25:
 
#Population model for flies [http://campus.mst.edu/ijde/contents/v8n2p1.pdf here]
 
#Population model for flies [http://campus.mst.edu/ijde/contents/v8n2p1.pdf here]
 
#[http://www.sciencedirect.com/science/article/pii/S089396591100468X this]
 
#[http://www.sciencedirect.com/science/article/pii/S089396591100468X this]
 +
 +
=References=
 +
* {{BookReference|Dynamic Equations on Time Scales|2001|Martin Bohner|author2=Allan Peterson|next=Forward jump}}: Theorem 1.16 (i)

Revision as of 06:05, 10 June 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

References