Computer Sciences > GATE 2024 Set-2 > Recurrence Relations
Let T(n) be the recurrence relation defined as follows:
T(0) = 1,
T(1) = 2, and
T(n) = 5T(n-1) - 6T(n-2) for n ≥ 2
Which one of the following statements is TRUE?
A
T(n) = Θ(2n)
B
T(n) = Θ(n2n)
C
T(n) = Θ(3n)
D
T(n) = Θ(n3n)

Correct : a

To find the correct asymptotic bound for T(n), let us solve the given linear homogeneous recurrence relation step-by-step using its characteristic equation.
1. Set up the Characteristic Equation: The recurrence relation for n ≥ 2 is:
T(n) = 5T(n-1) - 6T(n-2)

We assume a solution of the form T(n) = rn. Substituting this into the relation gives the quadratic characteristic equation:
r2 - 5r + 6 = 0
2. Find the Roots of the Equation: Factoring the quadratic polynomial:
(r - 2)(r - 3) = 0

The characteristic roots are r1 = 2 and r2 = 3.
3. Write the General Solution: Since the roots are real and distinct, the general solution is a linear combination of both roots raised to the power of n:
T(n) = c1(2n) + c2(3n)
4. Solve for Constants using Initial Conditions: We use the given base cases to evaluate c1 and c2:
• For n = 0:
    T(0) = c1(20) + c2(30) → c1 + c2 = 1
• For n = 1:
    T(1) = c1(21) + c2(31) → 2c1 + 3c2 = 2

Multiply the first equation by 2:
2c1 + 2c2 = 2

Subtracting this from the second equation:
(2c1 + 3c2) - (2c1 + 2c2) = 2 - 2 → c2 = 0

Substituting c2 = 0 back into the first equation:
c1 + 0 = 1 → c1 = 1
5. Find the Exact Expression and Asymptotic Complexity: Substituting our calculated constants c1 = 1 and c2 = 0 back into the general solution formula:
T(n) = 1 · (2n) + 0 · (3n) = 2n

Since the exact form reduces directly to 2n, its tight asymptotic bound is:
T(n) = Θ(2n)
Conclusion: The statement T(n) = Θ(2n) is true, matching option (a).

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