Covariance of Sums of Random Variables/Lemma
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Theorem
Let $n$ be a strictly positive integer.
Let $\sequence {X_i}_{1 \mathop \le i \mathop \le n}$ be a sequence of random variables.
Let $Y$ be a random variable.
Then:
- $\ds \cov {\sum_{i \mathop = 1}^n X_i, Y} = \sum_{i \mathop = 1}^n \cov {X_i, Y}$
Proof
Proof by induction:
For all $n \in \N$, let $\map P n$ be the proposition:
- $\ds \cov {\sum_{i \mathop = 1}^n X_i, Y} = \sum_{i \mathop = 1}^n \cov {X_i, Y}$
Basis for the Induction
We have that:
- $\ds \cov {\sum_{i \mathop = 1}^1 X_i, Y} = \cov {X_1, Y} = \sum_{i \mathop = 1}^1 \cov {X_i, Y}$
We therefore have that $\map P 1$ is true.
This is our basis for the induction.
Induction Hypothesis
Suppose that $\map P n$ is true for some fixed $n \in \N$.
That is:
- $\ds \cov {\sum_{i \mathop = 1}^n X_i, Y} = \sum_{i \mathop = 1}^n \cov {X_i, Y}$
We aim to show that it logically follows that $\map P {n + 1}$ is true.
That is:
- $\ds \cov {\sum_{i \mathop = 1}^{n + 1} X_i, Y} = \sum_{i \mathop = 1}^{n + 1} \cov {X_i, Y}$
Induction Step
This is our induction step:
We have:
\(\ds \cov {\sum_{i \mathop = 1}^{n + 1} X_i, Y}\) | \(=\) | \(\ds \cov {\sum_{i \mathop = 1}^n X_i + X_{n + 1}, Y}\) | splitting up the sum | |||||||||||
\(\ds \) | \(=\) | \(\ds \cov {\sum_{i \mathop = 1}^n X_i, Y} + \cov {X_{n + 1}, Y}\) | Covariance of Linear Combination of Random Variables with Another | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{i \mathop = 1}^n \cov {X_i, Y} + \cov {X_{n + 1}, Y}\) | Induction Hypothesis | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{i \mathop = 1}^{n + 1} \cov {X_i, Y}\) |
Hence the result by induction.
$\blacksquare$