3.7: Series
- Page ID
- 84694
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In mathematics, a series is the sum of the elements of a sequence. It’s a terrible name, because in common English, “sequence” and “series” mean pretty much the same thing, but in math, a sequence is a set of numbers, and a series is an expression (a sum) that has a single value. In math notation, a series is often written using the summation symbol \(\sum\).
For example, the sum of the first 10 elements of \(A\) is \[\sum_{i=1}^{10} A_i\notag\]
A for loop is a natural way to compute the value of this series:
Listing 3.1: A program that calculates a simple series
A1 = 1;
total = 0;
for i=1:10
a = A1 * (1/2)^(i-1);
total = total + a;
end
ans = totalLet’s walk through what’s happening here. A1 is the first element of the sequence, so we assign it to be 1; we also create total, which will store the cumulative sum. Each time through the loop, we set a to the \(i\)th element and add a to the total. At the end, outside the loop, we store total as ans
The way we’re using total is called an accumulator, that is, a variable that accumulates the result a little bit at a time.
This example computes the terms of the series directly. As an exercise, write a script named series.m that computes the same sum by computing the elements recurrently. You will have to be careful about where you start and stop the loop.