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Math and science::Analysis::Tao::07. Series

Series laws Ⅱ: sums over finite sets

9 basic properties of sums over finite sets. The proofs of these are a good exercise to refresh the notions of functions, bijections, sets, sums and induction.

There is 1-1 correspondence between many of these properties and the 6 properties already shown for sums of the form \( \sum_{i=1}^{n}a_i \). Thus, proving many of them amounts to choosing a bijection that allows transitioning to one of the original properties.

Question: when reading the below properties, consider:

  • what is the equivalent \( \sum_{i=1}^{n}a_i \) form, if it exists?
  • does the property extend to sums over infinite sets?

1. [...]. If \( X \) is empty, and \( f: X \rightarrow \mathbb{R} \) is a function (i.e., \( f \) is the empty function), we have

\[ \sum_{x \in X} f(x) = 0 \]

2. [...]. If \( X \) consists of a single element, \( X = \{x_0\} \), and \( f: X \rightarrow \mathbb{R} \) is a function, we have

\[ \sum_{x \in X}f(x) = f(x_0) \]

3. [...], part I. If \( X \) is a finite set, \( f: X \rightarrow \mathbb{R} \) is a function, and \( g: Y \rightarrow X \) is a bijection, then

\[ \sum_{x \in X}f(x) = \sum_{y \in Y}f(g(y)) \]

4. [...], part II. Let \( n \le m \) be integers, and let \( X \) be the set \( X := \{i \in \mathbb{Z} : n \le i \le m\} \). If \( a_i \) is a real number assigned to each integer \( i \in X \), then we have

\[ \sum_{i=n}^{m}a_i = \sum_{i \in X}a_i \]

5. [...]. Let \( X \), \( Y \) be disjoint finite sets (\( X \cap Y = \emptyset \)), and \( f: X \cup Y \rightarrow \mathbb{R} \) is a function. Then we have

\[ \sum_{z \in X \cup Y}f(z) = \left( \sum_{x \in X} f(x) \right) + \left( \sum_{y \in Y} f(y) \right) \]

6. [...], part I. Let \( X \) be a finite set, and let \( f: X \rightarrow \mathbb{R} \) and \( g: X \rightarrow \mathbb{R} \) be functions. Then we have

\[ \sum_{x \in X}(f(x) + g(x)) = \sum_{x \in X}f(x) + \sum_{x \in X}g(x) \]

7. [...], part II. Let \( X \) be a finite set, and let \( f: X \rightarrow \mathbb{R} \) be a function, and let \( c \) be a real number. Then

\[ \sum_{x \in X}cf(x) = c\sum_{x \in X}f(x) \]

8. [...]. Let \( X \) be a finite set, and let \( f: X \rightarrow \mathbb{R} \) and \( g: X \rightarrow \mathbb{R} \) be functions such that \( f(x) \le g(x) \text{ for all } x \in X \). Then

\[ \sum_{x \in X}f(x) \le \sum_{x \in X}g(x) \]

9. [...]. Let \( X \) be a finite set, and let \( f: X \rightarrow \mathbb{R} \) be a function, then

\[ |\sum_{x \in X}f(x)| \le \sum_{x \in X}|f(x)| \]