1.7 Proof by Induction 2

2026 Syllabus Objectives

  1. Use the method of mathematical induction to establish a given result (e.g., r3=14n2(n+1)2\sum r^3 = \frac{1}{4}n^2(n+1)^2, un=12(1+3n1)u_n = \frac{1}{2}(1 + 3^{n-1}) for the sequence given by un+1=3un1u_{n+1} = 3u_n - 1 and u1=1u_1 = 1, matrix powers, divisibility of 32n+2×5n33^{2n} + 2 \times 5^n - 3 by 8)

  2. Recognise situations where conjecture based on a limited trial followed by inductive proof is a useful strategy, and carry this out in simple cases (e.g., find the nth derivative of xexxe^x, find rr!\sum r \cdot r!)


🔑 What is Mathematical Proof?

Mathematical proof requires certainty that a relationship is true (or false) for all values being considered, without exception. While we might observe a pattern in a sequence like 1,2,4,8,16,1, 2, 4, 8, 16, \ldots where terms increase by a factor of 2 and appear to follow un=2nu_n = 2^n for n0n \geq 0, observation alone is not proof.

Mathematical induction provides a rigorous method to establish such results with certainty. It involves:

  • Setting up a mathematical inductive process using the first term (base case)
  • Using a general term to form a convincing argument for all values in an interval

📌 Key Terminology

Mathematical induction: A method of mathematical proof typically used to establish that a given statement is true for all natural numbers.

Conjecture: A mathematical statement which appears to be true, but has not been formally proven.

Base case (P1P_1 or P0P_0): The first step of an inductive proof, showing the statement holds for the initial value (e.g., n=1n=1 or n=0n=0).

Inductive step: The step in which you show that if the statement holds for n=kn=k, it must also hold for n=k+1n=k+1.

PkP_k: The statement being tested for a specific value kk.

Pk+1P_{k+1}: The statement for the next value in the sequence, k+1k+1.


⚡ The Principle of Mathematical Induction

The condition for proof by induction follows this structure:

If P1P_1 is true and PkPk+1P_k \Rightarrow P_{k+1}, then PnP_n is true for all n1n \geq 1.

This method can be applied to:

  • Summations of series
  • Derivatives of functions
  • Recurrence relations
  • Matrix powers
  • Divisibility problems

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