# Discrete math questions on relations and functions

Logic & Set Theory; Boolean Algebra; Relations & Functions

1. How do we distinguish relations from functions?

2. What sort of relation is friendship, using the human or sociological meaning of the word? Is it necessarily reflexive, symmetric, antisymmetric, or transitive? Explain why it is or is not any of these. What other types of interpersonal relationships share one or more of these properties? Explain.

3. Reduce the following Boolean product to zero OR a fundamental product: xyx'z.c

4. Write the dual of the following Boolean equation: a+a'b = a+b

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#### Solution Summary

The solution gives detailed steps on answering 4 discrete math questions on relations, functions, reflexive, symmetric, transitive, Boolean product and Boolean equation.

Discrete Mathematics and Functions

See the attached file.

1. Let f be any function from R to R. Define a relation Rf by the rule: x Rf y if and only if f(x) = f(y). Show that Rf is an equivalence relation. (Hint 1: first consider a simple specific case, such as f(x) = x2. That is, x and y are related if and only if x2 = y2. Then consider the general case. Hint 2: Think about contour lines on a map.)

2: For the special case f(x) = x2 of Problem 2, what are the equivalence classes? (Hint: Check out some special cases. Which numbers are related to 0? Which numbers are related to 7? Which numbers are related to -5?)

3: Consider three sets A, B, and C, where A has 7 elements, B has 5 elements, and C has 2 elements. How many (ternary) relations are there on the sets A, B, C?

4. Suppose that the directed graph below were the reduced graph of a partial order relation. What edges would you have to add to this graph to create the full graph of the partial order relation? (You will probably want to sketch this on paper, but in responding, you don't have to provide an illustration, just list the edges to be added.)

5. Define a relation on the squares of a chessboard by saying that two squares are related if an only if a bishop can get from one square to the other by a sequence of legal moves? This is clearly an equivalence relation. How many equivalence classes are there?

Describe them.

(Info for non-chess players: a chessboard is the same thing as a checkerboard and is 8 x 8. A bishop can only move diagonally.)

Question for discussion: You may have noticed that I have freely batted around the word counterexample. What does this word mean? Can you give some examples of counterexamples from everyday life?

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