Let ๐‘‡(๐‘›) be the recurrence relation defined as follows: ๐‘‡(0) = 1, ๐‘‡(1) =โ€ฆ

2024

Let ๐‘‡(๐‘›) be the recurrence relation defined as follows:

ย ย ย ย ย ย ย ย ๐‘‡(0) = 1,

ย ย ย ย ย ย ย ย ๐‘‡(1) = 2, and

ย ย ย ย ย ย ย ย ๐‘‡(๐‘›) = 5๐‘‡(๐‘› โˆ’ 1) โˆ’ 6๐‘‡(๐‘› โˆ’ 2) for ๐‘› โ‰ฅ 2

Which one of the following statements is TRUE?

  1. A.

    ๐‘‡(๐‘›) = ฮ˜(2๐‘› )

  2. B.

    ๐‘‡(๐‘›) = ฮ˜(๐‘›2๐‘› )

  3. C.

    ๐‘‡(๐‘›) = ฮ˜(3๐‘›)

  4. D.

    ๐‘‡(๐‘›) = ฮ˜(๐‘›3๐‘›)

Attempted by 85 students.

Show answer & explanation

Correct answer: A

Key idea: solve the recurrence by the characteristic equation.

  • Form the characteristic equation: r^2 - 5r + 6 = 0, which has roots r = 2 and r = 3.

  • Write the general solution: T(n) = Aยท2^n + Bยท3^n.

  • Use initial conditions: T(0)=1 โ‡’ A + B = 1; T(1)=2 โ‡’ 2A + 3B = 2.

  • Solve the system: subtracting 2ร—(A + B = 1) from (2A + 3B = 2) gives B = 0, so A = 1.

  • Therefore T(n) = 2^n exactly, so T(n) = ฮ˜(2^n).

Conclusion: the recurrence solves to T(n) = 2^n, hence the correct growth is ฮ˜(2^n).

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