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Discrete Mathematics Demystified

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CHAPTER 13<br />

Series<br />

13.1 Fundamental Ideas<br />

The idea of adding together finitely many numbers is a simple and familiar one.<br />

We are so used to the associative and commutative properties of finite sums that<br />

we tend not to think about them. Now we shall learn to add together infinitely many<br />

numbers, and we will have to be a bit more careful. As Example 13.1 suggests,<br />

contradictions can arise if we do not establish certain ground rules.<br />

EXAMPLE 13.1<br />

Give an example of an infinite summation process in which the ordinary rules of<br />

arithmetic appear to fail.<br />

Solution: Consider the infinite sum<br />

If we group the terms as<br />

1 − 1 + 1 − 1 + 1 − 1 + 1 − 1 +···<br />

(1 − 1) + (1 − 1) + (1 − 1) + (1 − 1) +···

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