The number of different words that can be formed with 12 consonants and 5 vowels

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Consider an alphabet $A$ consisting of $6$ vowels and of $12$ consonants. Valid words consist of no two consonants in a row, so AART is not valid, nor is JUDITH, but JUDIT is fine and so is AAR, as is AIAIAIAIAIAIAIAIAI. $a_n$ denotes the amount of valid words.

a) find $a_0$, $a_1$, $a_2$, $a_3$

$a_0=1$, the empty word

$a_1=12+6=18$ (just one letter)

For $a_2$ we considers words like $AT$, $TA$, $IA$(different vowels) and $AA$ (same vowels)

$a_2= 2 \times 6 \cdot 12 + 5 \cdot 6 + 6=144 +30 +6=180$

We expand to three symbols by either adding a vowel to the end of a 2-letter word or by adding a vowel and consonant to a 1-letter word

$a_3=180 \cdot 6 + 6 \cdot 12 \cdot 18 =1080+1296=2376$

(b) Find a recurrence relation

We make a case distinction for a valid word of length $n$, it either ends in a consonant or in a vowel. If it ends in a consonant, we must have obtained it from a valid word of length $n-2$ by placing a vowel followed by a consonant behind it. In all other situations we simply place a vowel behind a word of length $n-1$.

We get for $n\geq 2$: $$ a_n = 6 \cdot a_{n-1} + 6 \cdot 12 \cdot a_{n-2}$$ One can verify that this indeed gives $180$ for $a_2$.

We can solve this recursion via an auxiliary equation of the form:

$$ r^2 = 6r + 6 \cdot 16 $$ $$ r^2 - 6r - 6 \cdot 16 =0$$ Which factorises as:

$$ (r-12)(r+6)=0$$

So we get solutions $a_n = A r_1^n + B r_2^n$:

$$ a_n = A \cdot 12^n + B \cdot (-6) ^n$$

We can now plug in our initial conditions $a_0=1$ and $a_1=18$ $$1=A+B$$ $$ 18= 12A - 6B=18A -6 \implies 18A=24 \implies A=\frac{4}{3}, B=-\frac{1}{3}. $$

We get:

$$ a_n = \frac{4}{3}\cdot 12^n -\frac{1}{3} (- 6)^n$$

I feel that this is probably correct, but I am unsure. Can someone please verify?

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English

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Chapter

Permutation And Combination

find the number of different w...

Updated On: 27-06-2022

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