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1.5.2 Proof by Mathematical Induction

Basics of Mathematical Induction

Mathematical induction serves as a pivotal proof technique, especially in number theory, algebra, and combinatorics, to establish the veracity of statements defined for all natural numbers. For a deeper understanding, you may review the direct and indirect proofs that form the foundation of proof techniques.

Principle of Mathematical Induction

The principle of mathematical induction is a method used to prove a statement, often denoted as P(n), for all natural numbers n. The principle is based on two steps:

  • Base Case: Validate the statement for the initial integer, commonly 1.
  • Inductive Step: Assume the statement is true for an arbitrary integer k and prove it for k + 1. This method can be particularly useful when dealing with arithmetic sequences.

Importance in Mathematics

  • Infinite Implications: Induction allows mathematicians to prove statements for infinitely many cases with finite steps.
  • Structural Simplicity: Despite its powerful implications, the method retains a straightforward and systematic structure. It's often applied alongside other techniques such as those used in the basics of binomial expansion.

Historical Context

The principle of mathematical induction has been a proven method and is widely acknowledged in various mathematical principles and solved mathematics problems. It is also referred to as the method of induction or simply the principle of induction.

Proving Sequences Using Induction

Sequences often present properties or formulas that are claimed to hold for all natural numbers, making induction a suitable proof method. For more examples, consider exploring exponential equations where similar techniques are applied.

Example: Arithmetic Sequence Sum Formula

Statement: The sum S of the first n terms of an arithmetic sequence with first term a and common difference d is given by: S = n/2 (2a + (n-1)d)

Proof by Induction

  • Base Case: For n=1, S = a, which satisfies the formula.
  • Inductive Step: Assume true for n=k and prove for n=k+1.

Assumption: Sk = k/2 (2a + (k-1)d)

To Prove: S(k+1) = (k+1)/2 (2a + k*d)

Proof: S(k+1) = Sk + a + kd Substituting the assumed formula for Sk: S(k+1) = k/2 (2a + (k-1)d) + a + kd = (k+1)/2 (2a + k*d)

Thus, the formula is proven true for n=k+1, validating the statement for all n using mathematical induction.

Proving Inequalities Using Induction

Inequalities that are proposed to be true for all natural numbers can be substantiated using induction. You may also find it helpful to study related topics such as the binomial distribution.

Example: Inequality of Factorials

Statement: For all integers n >= 4, n! > 2n.

Proof by Induction

  • Base Case: For n=4, 4! = 24 > 16 = 24, which is true.
  • Inductive Step: Assume true for n=k and prove for n=k+1.

Assumption: k! > 2k

To Prove: (k+1)! > 2(k+1)

Proof: (k+1)! = (k+1) * k! Using the assumption and noting that 2 * 3(2k) is even:

(k+1) * 2k Since k >= 4, k+1 > 2, and thus: 2(k+1)

This establishes the truth of the statement for n=k+1, proving the inequality for all n using induction.

Proving Divisibility Using Induction

Induction can validate divisibility properties that are presumed to be true for all natural numbers. To further understand the proofs, you can look into the direct and indirect proofs discussed earlier.

Example: Divisibility Property

Statement: 3(2n) - 1 is divisible by 8 for all n in N.

Proof by Induction

  • Base Case: For n=1, 32 - 1 = 8, which is divisible by 8.
  • Inductive Step: Assume true for n=k and prove for n=k+1.

Assumption: 3(2k) - 1 is divisible by 8

To Prove: 3(2(k+1)) - 1 is divisible by 8

Proof: 3(2(k+1)) - 1 = 3(2k) * 32 - 1 = (3(2k) - 1) + 2 * 3(2k) Using the assumption and noting that 2 * 3(2k) is even: is divisible by 8

This proves the statement for n=k+1, validating the divisibility property for all n using induction.

The above examples illuminate the application of mathematical induction in proving various types of statements, from sequences and inequalities to divisibility properties, providing a robust tool for mathematicians to validate infinite cases in a finite manner. This method not only enhances logical reasoning but also fortifies foundational understanding, enabling students to construct robust mathematical arguments and solve complex problems with finesse.

FAQ

Mathematical induction is a method used for proving statements, not disproving them. However, if during the process of trying to prove a statement using induction a contradiction or inconsistency is found, this could indicate that the statement is false. To disprove a statement, you would typically provide a counterexample – a specific case that contradicts the statement. It's essential to approach mathematical statements with a neutral stance, seeking to either prove or disprove them based on logical reasoning and evidence.

No, a false hypothesis in mathematical induction invalidates the argument. The principle of mathematical induction relies heavily on the hypothesis being true for an arbitrary integer k to establish its truth for k+1. If the hypothesis (P(k) is true) is false, then the subsequent argument (P(k+1) is true) is not reliably established, and the proof is invalid. It’s crucial to ensure the hypothesis is true to maintain the logical integrity of the proof and accurately extend the truth of the statement to all relevant integers.

The base case serves as the foundational step in a proof by mathematical induction. It verifies the statement for the initial value (often n=1), ensuring that the first domino in the metaphorical line of dominoes falls. Without establishing the truth of the base case, the inductive step, which assumes the statement is true for an arbitrary k and proves it for k+1, cannot be initiated. Essentially, the base case acts as the starting point, and without it, the inductive step cannot perpetuate the truth of the statement to other integers in the set.

The assumption that P(k) is true in the inductive step, known as the inductive hypothesis, is a fundamental component of mathematical induction. This assumption is used to prove that P(k+1) is true, thereby showing that the truth of P(k) implies the truth of P(k+1). This creates a domino effect: since P(1) is true (established in the base case), P(2) must be true, which implies P(3) is true, and so on, thereby proving P(n) is true for all positive integers n. The method leverages this chain of implications to establish the universal truth of the statement without having to prove it directly for each individual case.

Mathematical induction is primarily used to prove statements about integers, especially natural numbers. The method relies on the well-ordered principle of natural numbers, which states that every non-empty set of positive integers has a least element. This principle doesn’t hold for real or rational numbers, which is why mathematical induction is not typically used for these sets. However, it's worth noting that certain properties or statements that are proven true for all natural numbers using induction might also be true for real or rational numbers, but additional justification or a different proof method would be needed to establish these results.

Practice Questions

Prove that the sum of the first n odd numbers is equal to n^2, for all positive integers n.

The statement we want to prove by induction is P(n): 1 + 3 + 5 + ... + (2n-1) = n2.

  • Base Case: When n=1, the sum of the first odd number (1) is equal to 12, which is true.
  • Inductive Step: Assume P(k) is true for some arbitrary positive integer k, i.e., 1 + 3 + 5 + ... + (2k-1) = k2. We need to prove P(k+1), i.e., 1 + 3 + 5 + ... + (2k-1) + (2k+1) = (k+1)2.

Using the assumption P(k), we can write P(k+1) as: k2 + (2k+1). Factoring the right side of P(k+1), we get: k2 + 2k + 1 = (k+1)2. Since k2 + (2k+1) equals (k+1)2, P(k+1) is true whenever P(k) is true. Therefore, by the principle of mathematical induction, P(n) is true for all positive integers n.

Prove that 2^n > n^2 for all integers n > 4.

To prove this statement, we will use the principle of mathematical induction.

  • Base Case: When n=5, 25 = 32 and 52 = 25. Thus, 32 > 25, and the statement holds for n=5.
  • Inductive Step: Assume the statement is true for some arbitrary positive integer k, i.e., 2k > k2. We need to prove that 2(k+1) > (k+1)2.

Using the assumption 2k > k2, we can say that 2(k+1) = 2 * 2k > 2 * k2. Now, we need to show that 2 * k2 > (k+1)2. Expanding (k+1)2, we get k2 + 2k + 1. Since k is greater than 4, 2 * k2 is certainly greater than k2 + 2k + 1. Therefore, 2(k+1) > (k+1)2 whenever 2k > k2. By the principle of mathematical induction, 2n > n2 is true for all integers n > 4.

These questions and solutions demonstrate the application of the principle of mathematical induction to prove statements involving inequalities and series. The solutions are structured to first establish the base case and then use the inductive hypothesis to prove the next step, thereby validating the statement for all relevant integers. This logical and systematic approach is crucial in constructing mathematical proofs and is a fundamental skill in IB Mathematics.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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