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My textbook, Statistical Inference, Second Edition, by Casella and Berger, provides the following theorem (the Fundamental Theorem of Counting) and accompanying proof:

Theorem 1.2.14 If a job consists of $k$ separate tasks, the $i$th of which can be done in $n_i$ ways, $i = 1, \dots, k$, then the entire job can be done in $n_1 \times n_2 \times \dots \times n_k$ ways.

Proof: It suffices to prove the theorem for $k = 2$ (see Exercise 1.15). The proof is just a matter of careful counting. The first task can be done in $n_1$ ways, and for each of these ways we have $n_2$ choices for the second task. Thus, we can do the job in

$$\underbrace{(1 \times n_2) + (1 \times n_2) + \dots + (1 \times n_2) = n_1 \times n_2}_\text{$n_1$ terms}$$

ways, establishing the theorem for $k = 2$. $\tag*{$\square$}$

Am I correct in thinking that this is not a valid (general) proof of the theorem? It is only a proof for $k = 2$, which, as I understand it, and contrary to what the author says, is not sufficient to prove the theorem.

I would appreciate it if people would please take the time to clarify this.

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  • $\begingroup$ This proof is valid, and you can simply treat completing task #$1$,..., task #$k-1$ as one single task altogether. $\endgroup$ – Clement Yung Dec 26 '19 at 13:20
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    $\begingroup$ What does exercise 1.15 say? $\endgroup$ – Angina Seng Dec 26 '19 at 13:21
  • $\begingroup$ @LordSharktheUnknown I haven't checked (exercises are at the end of the chapter). EDIT: "Finish the proof of Theorem 1.2.14. Use the result established for $k = 2$ as the basis of an induction argument." Haha, okay, that clarifies things. Thanks for that. $\endgroup$ – The Pointer Dec 26 '19 at 13:21
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Since the number of tasks is finite, then it is a valid proof. If you have 3 tasks $a,b,c$ then you can see $\{a,b\}$ for example as one task and $c$ as a "second" task. So what you proved for $k=2$ will still work for $3$ and so on ...

It is similar to the idea of induction

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    $\begingroup$ To make the proof precise one does use induction. $\endgroup$ – Berci Dec 26 '19 at 14:17

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