Recursion (Cont.)

Algorithms - CS2500 - 25 January 2018


A proof is a sequence of steps to get from a hypothesis to a conclusion


An example that proves a certain point is wrong.

###The principle of Recursion Definition: A property P on a domain D is a function P that accepts inputs from the domain D and returns a Boolean value. If P is a property on D, and d is in D then we say that P holds for d if P(d) is true, P does not hold for d if P(d) is false

Ex: D is the set of all cars and P is a property “is blue”

The Six Steps:

  1. Define the Problem
  2. Check Stopping Values + Two More
  3. Check that Recursion Stays in D
  4. Check that Recursion Halts
  5. Check Inheritance
  6. Conclude the proof

Step 1. Define the Problem

  • Define the Domain D: clearly state what objects you are dealing with – numbers, trees, Tower of Hanoi disks, etc.
  • Provide names and definitions for all functions you will be talking about
    • You can often use Python for the definitions
    • Look carefully for anomalies such as division by 0
  • State clearly what you are trying to prove.

Step 2. Check the Stopping Values and Two Other Values

  • Checking that whatever you are trying to prove is required for the stopping values, but optional for the two other values
  • We do the two other values because we want to reduce the likelihood of having made a mistake
  • If you fail, go back to Step 1.

Step 3. Prove that Recursion Stays in D

  • If you fail, go back to Step 1.

Step 4. Prove that Recursion Halts

  • Don’t trace the recursion
  • We use the Counting Strategy
  • Assign a non-negative integer as a counter to every value in the domain D. If whenever recursion is triggered, the counters associated with called values are < the value associated with the calling value, recursion will halt!
  • If you fail, go back to Step 1.

Step 5. Check that P is Inherited Recursively

  • This is often the hard part
  • You may assume that P is true for all values called recursively.
  • If you fail, go back to Step 1.

Step 6. Invoke the Principle of Recursion

  • Clearly indicate that you have finished the proof.

Equivalent Functions

  • How can you tell that 2 functions are always equivalent?

Example Proof:

  1. Domain D: All natural numbers; P(n) is true if f1(n) = f2(n); We want to prove that for any natural number n, P(n) is true.
  2. Check the values
  3. Use the counting strategy. Use n as the counter, the counter decreases, and done.
  4. Statement: f2(n) = n + f2(n-1); f2(n) = n + f1(n-1); f2(n) = n + n*(n-1)/2; n*(1 + (n-1)/2); n*(2 + (n-1))/2; n*(n+1)/2; f1(n); Therefore: f2(n) = f1(n)


  • P mod Q is always between 0 and Q-1 (assuming P and Q are positive integers)
  • (P+R) mod Q = P mod Q+R mod Q