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Consider the sequence S0,S1,S2,.... defined as follows: S0 = 0, S1 = 1 and Sn =  2Sn-1 + Sn-2 for n > 2. Which of the following statements is FALSE?
  • a)
    for every n > 1, S2n is even
  • b)
    for every n > 1, S2n+1 is odd
  • c)
    for every n > 1, S3n is multiple of 3
  • d)
    for every n > 1, S4n  is multiple of 6
  • e)
    none of the above
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
Consider the sequence S0,S1,S2,....defined as follows: S0 = 0, S1 = 1 ...
Sn = 2Sn−1 + Sn−2
Characterstic polynomial for this recurrence is x2 = 2x + 1
x2 − 2x − 1 = 0 
The solution to the recurrence relation is of the form  :

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Most Upvoted Answer
Consider the sequence S0,S1,S2,....defined as follows: S0 = 0, S1 = 1 ...
Explanation:
To determine which statements are true or false, we need to examine the given sequence and analyze the properties of the terms.

Sequence Definition:
The sequence is defined as follows:
- S0 = 0
- S1 = 1
- Sn = 2Sn-1 + Sn-2 for n ≥ 2

Statement a: for every n ≥ 1, S2n is even
To prove this statement, we need to show that S2n is always divisible by 2 for any positive value of n.

Proof by Induction:
Base Case:
For n = 1, S2 = S1 = 1, which is odd.
So, the base case is not divisible by 2.

Inductive Hypothesis:
Assume that for some positive integer k, S2k is divisible by 2.

Inductive Step:
We need to prove that S2(k+1) is divisible by 2.
S2(k+1) = 2S2k + S2(k-1)
Since S2k is divisible by 2 (as per our assumption), and S2(k-1) is also divisible by 2 (as per the inductive hypothesis), the sum of two even numbers is always even.
Therefore, S2(k+1) is divisible by 2.

By the principle of mathematical induction, we can conclude that for every n ≥ 1, S2n is even.

Statement b: for every n ≥ 1, S2n + 1 is odd
To prove this statement, we need to show that S2n + 1 is never divisible by 2 for any positive value of n.

Proof by Contradiction:
Assume that there exists some positive integer k for which S2k + 1 is divisible by 2.
This means S2k + 1 = 2m for some integer m.

Substituting the recursive definition of the sequence:
2S2(k-1) + S2(k-2) + 1 = 2m
2(S2(k-1) + S2(k-2)) + 1 = 2m
2(S2(k-1) + S2(k-2)) = 2m - 1

The left-hand side of the equation is even (as it is a multiple of 2), while the right-hand side is odd (as it is 1 less than a multiple of 2).
This leads to a contradiction, proving that S2k + 1 is never divisible by 2.

Therefore, for every n ≥ 1, S2n + 1 is odd.

Statement c: for every n ≥ 1, S3n is a multiple of 3
To prove this statement, we need to show that S3n is always divisible by 3 for any positive value of n.

Counterexample:
Let's consider n = 1:
S3(1) = S3 = 2S2 + S1 = 2(1) + 1 = 3
3 is not divisible by 3, so the statement is false.

Therefore, statement c is false.

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