## What is a (mathematical) sequence?

A sequence is an ordered list of numbers. It can be understood as a function that maps a real number to every natural number. So for every natural number starting from 1 (or 0 sometimes), the function associates a real number. Consider the following example sequence:

In this example, 1 is the first term in the sequence, 3 is the second term and so on. If we express this sequence as a function , we get for the terms above:

(1)

We can calculate the th term in this example as . So in this case, the function is and indeed we if we substitute 1 to 5 for , we obtain the same numbers as above.

### Example of a sequence

Consider a sequence . To find the 10th term () in this sequence, we substitute 10 for :

## Sequences in recursive and explicit forms

There are two ways to define a sequence. In the example above we used an *explicit form*. This means that we can directly calculate the th term. The *recursive form* of a sequence describes how the next term can be obtained if the previous term is known. One term of the sequence must be given as well as rule how to obtain the next term from it. The same sequence of our example can be defined recursively as:

(2)

In this case, if we want to find the 10th term, we must start with the first which is given by the definition (2 in this case). Then we can obtain the next term which is . We always find the next term by multiplying the previous term by 2 until we find the 10th term which is also 1024 as in our example before.

### Example: Fibonacci sequence

One famous sequence is called the Fibonacci sequence and is defined recursively as follows:

(3)

In this case, the recursive rule requires two previously known terms to calculate the next term. This is the reason why two terms are given in the definition. We can apply the rule to find the terms of the sequence which gives us:

### Example: sequence of prime numbers

Some sequences do not have a rule to calculate a term explicitly or recursively. One example is the sequence of all prime numbers. The first few terms of the sequence are:

To find the other terms of the sequence, we need to analyse every number to see if it is a prime number and part of the sequence or not.