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Suppose that each pair of a genetically engineered species of rabbits left on an island produces two new pairs of rabbits at the age of 1 month and six new pairs of rabbits at the age of 2 months and every month afterward. None of the rabbits ever die or leave the island. a) Find a recurrence relation for the number of pairs of rabbits on the island n months after one newborn pair is left on the island. b) By solving the recurrence relation in (a) determine the number of pairs of rabbits on the island n months after one pair is left on the island.

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(a) Derivation recurrence relation\textbf{Derivation recurrence relation}

Let ana_n represent the number of pairs of rabbits on the island after nn months.

First case\textbf{First case} The rabbits that were present on the island in the previous (n1)(n-1)th month will remain on the island and will not die. Since there are an1a_{n-1} pairs of rabbits on the island in the previous month, these an1a_{n-1} pairs are still on the island.

an1a_{n-1}

Second case\textbf{Second case} The pairs of rabbits that were born in the previous (n1)(n-1)th month each had 2 pairs of rabbits. Since there were an2a_{n-2} pairs of rabbits 2 months ago and since there were an1a_{n-1} pairs of rabbits 1 month ago, an1an2a_{n-1}-a_{n-2} pairs of rabbits were born 2 months ago.

2(an1an2)2(a_{n-1}-a_{n-2})

Third case\textbf{Third case} The pairs of rabbits that were born two months ago (n2n-2) or earlier, each had 6 pairs of rabbits. Since there were an2a_{n-2} pairs of rabbits 2 months ago, each of these an2a_{n-2} pairs had 6 pairs of rabbits.

6an26a_{n-2}

Add all number of pairs for each case:

an=an1+2(an1an2)+6an2=3an1+4an2a_n=a_{n-1}+2(a_{n-1}-a_{n-2})+6a_{n-2}=3a_{n-1}+4a_{n-2}

Initial conditions\textbf{Initial conditions}

At 0 months, 1 newborn pair of rabbits is left on the island:

a0=1a_0=1

At 1 month, the one-month-old pair of rabbits had 2 new pairs of rabbits and thus there are then 1+2=31+2=3 pairs of rabbits on the island:

a1=3a_1=3

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