# Practice Paper 3 Question 14

The *Seesat* language has the single-letter word \(A.\) Longer words are built by applying a sequence of the following rules: Rule \(r_1\) says that if \(x\) is a word then \(Ax\) is a word; Rule \(r_2\) says that if \(x\) and \(y\) are words then \(Bxy\) is a word. For example, the sequence \(r_1(r_2(A,A))\) yields the word \(ABAA.\)

- Give a valid sequence of rules that yields the word \(AAABBAAA.\)
- Write \(N_A (x)\) and \(N_B(x)\) for the number of \(A\) and \(B\) letters in the word \(x\) respectively. By expressing \(N_A(x)\) and \(N_B(x)\) in connection with the rules, prove mathematically that \(N_A(x) > N_B(x)\) for any word \(x\) in the language.

## Related topics

## Warm-up Questions

- Prove, by mathematical induction, that \(\sum_{k=1}^{n}k^2 = \frac{n(n+1)(2n+1)}{6}.\)

## Hints

- Hint 1In simple terms, what do rule \(r_1\) and \(r_2\) do?
- Hint 2How many times should you apply rule \(r_2\) to get \(AAABBAAA?\)
- Hint 3If \(x\) is a word in the language, express \(N_A(r_1(x))\) in terms of \(N_A(x).\)
- Hint 4Similarly, find \(N_B(r_1(x)),\) \(N_A(r_2(x,y))\) and \(N_B(r_2(x,y)),\) with \(y\) being another word in the language.
- Hint 5What assumption do you need to make in order to claim that \(N_A(r_1(x)) > N_B(r_1(x))\) and \(N_A(r_2(x,y)) > N_B(r_2(x,y))?\)
- Hint 6Is your assumption valid?
*Hint:*Consider \(N_A(A)\) and \(N_B(A).\) - Hint 7Try proving by induction.
- Hint 8... more specifically, induction over each of the rules.

## Solution

Starting with a single-letter word \(A,\) the rules allow us to create new words from existing ones. Rule \(r_1\) lets us add an \(A\) in front of a word, while rule \(r_2\) combines two words and adds a \(B\) in front. There are \(2\) \(B\)s in \(AAABBAAA,\) thus we must apply rule \(r_2\) twice: first to have \(BAA,\) then \(BBAAA.\) Hence, the sequence of rule is \(r_1(r_1(r_1(r_2(r_2(A,A),A)))).\)

In order to prove that \(N_A(x) > N_B(x)\) for any word \(x\) in the language, we use induction over each of the rules. The base case is simple, \(N_A(A) = 1 > 0 = N_B(A).\) For the inductive step, our assumption is that \(N_A(x) > N_B(x)\) and \(N_A(y) > N_B(y),\) for some words \(x\) and \(y\) in the language. We have to show that \(N_A(r_1(x)) > N_B(r_1(x))\) and \(N_A(r_2(x,y)) > N_B(r_2(x,y)):\)

\(r_1:\) \(N_A(r_1(x)) = 1 + N_A(x) > N_B(x) = N_B(r_1(x))\)

\(r_2:\) \(N_A(r_2(x,y)) = N_A(x) + N_A(y) > 1 + N_B(x) + N_B(y) = N_B(r_2(x,y)).\)

Hence, \(N_A(x) > N_B(x)\) for all words \(x\) in the language.

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