Mathematical Induction Math Example 2

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Example 2

hard
Prove by induction: n!>2nn! > 2^n for all integers nβ‰₯4n \ge 4.

Solution

  1. 1
    Base case (n=4n = 4): 4!=244! = 24 and 24=162^4 = 16. Since 24>1624 > 16, the base case holds.
  2. 2
    Inductive hypothesis: Assume k!>2kk! > 2^k for some kβ‰₯4k \ge 4.
  3. 3
    Inductive step: (k+1)!=(k+1)β‹…k!>(k+1)β‹…2k(k+1)! = (k+1) \cdot k! > (k+1) \cdot 2^k. Since kβ‰₯4k \ge 4, we have k+1β‰₯5>2k+1 \ge 5 > 2.
  4. 4
    Therefore (k+1)!>2β‹…2k=2k+1(k+1)! > 2 \cdot 2^k = 2^{k+1}. The inequality holds for k+1k+1.

Answer

n!>2nΒ forΒ allΒ nβ‰₯4n! > 2^n \text{ for all } n \ge 4
In the inductive step, we multiply both sides by (k+1)(k+1) and use the fact that k+1>2k+1 > 2 to strengthen the bound. The base case starts at n=4n=4 because the claim is false for n≀3n \le 3.

About Mathematical Induction

Mathematical induction proves statements indexed by integers by verifying a base case and an inductive step.

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