Department of Mathematics

Colorado State University

Math 301: Induction Practice

Class date: Friday, September 6, 2019
  1. Use induction to prove that \[ 1 + 3 + 5 + \cdots + (2n - 1) = n^2.\]

    1. First, prove the base case, where \(n = 1\).

    2. Second, suppose the inductive hypothesis that for any \(k \in \mathbb N\), we know that \[ 1 + 3 + 5 + \cdots + (2k - 1) = k^2.\] Using this, show that \[ 1 + 3 + 5 + \cdots + (2k - 1) + (2(k+1) - 1) = (k+1)^2.\]

    3. Conclude: “By the principle of induction, we conclude that \[ 1 + 3 + 5 + \cdots + (2n - 1) = n^2.\] holds for any natural number \(n\). \(\Box\)”

    We prove this by induction.

    Base case: Take \(n=1\), and notice that \(1 = 1^2\).

    Inductive step: Suppose as an inductive hypothesis that for \(n = k\) with \(k \ge 1\), \[ 1 + 3 + 5 + \cdots + (2k - 1) = k^2. \]

    (Remark: We aim to show that the statement holds for \(n = k+1\), that is that \(1 + 3 + 5 + \cdots + (2k - 1) + (2(k+1) - 1) = (k+1)^2.\))

    We’ll use algebra to show that the LHS of our desired equality is equal to the RHS:

    By the transitive property of equality, we have shown that \[ 1 + 3 + 5 + \cdots + (2k - 1) + (2(k+1) - 1) = (k+1)^2. \]

    Hence, by the principle of induction, the statement is true for all \(n \in \mathbb N\).

  2. Use induction to prove that \[ n! \ge 2^n \] for all \(n \ge 4\).

    1. First, prove the base case. Notice that \(n \ne 1\) for this base case!

    2. Complete the proof.

    We prove this by induction.

    Base case: Take \(n=4\), and notice that \(4! = 24 \ge 16 = 2^4\).

    Inductive step: Suppose as an inductive hypothesis that for \(n = k\), with \(k \ge 4 \); \[ k! \ge 2^{k}. \]

    (Remark: We aim to show that the statement holds for \(n = k+1\), that is that \( (k+1)! \ge 2^{k+1}.\))

    We’ll use algebra to show that the LHS of our desired inequality is larger than the RHS.

    By the transitive property of \(\ge\), we have shown that \[ (k+1)! \ge 2^{k+1}. \]

    Hence, by the principle of induction, the statement is true for all \(n \in \mathbb N\) when \(n \ge 4\).

  3. Use induction to prove that there are \(n!\) different ways to order the numbers of the set \(\{ 1, 2, 3, \cdots, n \}\). (Hint. If I have a shuffled deck of cards, and I wanted to put a new card in the deck, how many ways are there for me to do this?)

    We prove this by induction.

    Base case: Take \(n=1\), and notice that there is only one way to order the set, \(1\).

    Inductive step: Suppose as an inductive hypothesis that for \(n = k \ge 1\), there are \(k!\) ways to order the set.

    Consider the set \(S = \{ 1, 2, 3, \cdots, k, k+1 \}\). There are \(k!\) ways to order the set \(S \setminus \{k+1\}\); to each of these there are \((k+1)\) places to put the element \(k+1\) in an ordering of \(S\). So, there are \((k+1)k! = (k+1)!\) ways to order the set \(S\).

    Hence, by the principle of induction, the statement is true for all \(n \in \mathbb N\).