Sequences |

**In General**

$ __Def__: A sequence in
a set *X* is a subset *A* ⊂ *X* and
a map \(\mathbb N\) → *A*, *n* \(\mapsto\) *a _{n}*,
which is onto, so that

>

**Operations on Sequence**s

$ __Difference operator__:
The mapping {*a*_{n}}
→ {*b*_{n}} defined by *b*_{n} =
*a*_{n+1} – *a*_{n},
for all *n*.

**Types of Sequences**

$ __Cauchy sequence__: A
sequence {*a*_{n}}
in a normed space such that, for all *ε* > 0, ∃ *N* such
that for all *n*, *m* > *N*, || *a*_{n}–*a*_{m}|| < *ε*.

$ __Monotonic sequence__: (Increasing)
One such that the difference operator produces a non-negative sequence.

$ __Completely monotonic sequence__:
(Increasing) One such that *k* iterations of the difference operator produce
a non-negative sequence, for all *k*.

**Convergence** > s.a. limit;
series.

$ __In a topological space
( X, T)__: A sequence {

$

$

$

**Fibonacci Sequence** > s.a. Golden Ratio.

$ __Def__:
The sequence 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, ..., in which *F*_{n} =
*F*_{n–2} + *F*_{n–1};
It appears in life patterns.

* __Result__: (Zeckendorf) Every integer can be written uniquely as a sum of non-consecutive Fibonacci numbers.

@ __General references__: Vorobiev 02 [IIb]; Posamentier & Lehmann 07; Miller & Wang JCTA(12) [and central-limit-type theorems].

@ __In nature__: news pw(07)apr [stress-related
Fibonacci spirals]; Pennybacker & Newell PRL(13) [underlying physical and biochemical mechanisms in plant growth].

> __Online resources__: See Wikipedia page.

**Other Examples** > see Wikipedia page on integer sequences.

**Generalizations** > s.a. projective family.

* __Inductive family or system__:
Given a directed set *I*, an inductive
family on *I* is a collection {*X*_{i} | *i* ∈ *I*}
of objects in a category, and immersions
{*φ*_{ij}: *X*_{i} → *X*_{i} |
*i* ≥ *j* ∈ *I*},
such that *φ*_{ii} =
id_{Xi},
and *φ*_{ij} \(\circ\) *φ*_{jk} =
*φ*_{ik}; __Remark__:
In many (all?) categories each such family defines an inductive limit.

main page – abbreviations – journals – comments – other
sites – acknowledgements

send feedback and suggestions to bombelli at olemiss.edu – modified 26
feb
2017