Mathematical Logic

TEXT IN RED IS INCORRECT

Tautologies and Contradictions

A proposition that is always true is called a tautology. A proposition that is always false is called a contradiction. To test if a given proposition is a tautology, a contradiction or neither we make for the proposition. If all truth values in the are T's then the proposition is a tautology, if they are all F's then it is a contradiction, otherwise it is neither a tautology nor a contradiction.

Example 1. The proposition pp is a tautology.

Example 2. The proposition (pq)(pq) is a contradiction.

 

Logical Equivalence

Two propositions whose truth tables have the same last column are called logically equivalent. This means that no matter what truth values the primitive propositions have, these two propositions are either both true or both false. To test whether or not two propositions are logically equivalent we make a truth table for each of them and compare their last columns. If they are then the two propositions are logically equivalent, otherwise they are not.

Example: The proposition (pq)r is logically equivalent to p(qr). This is why the expression pqr is , wherever the parentheses go the result is equivalent.

Similarly, the proposition (pq)r is logically equivalent to p(qr) so the expression pqr is . Wherever the parentheses go the result is equivalent.

De Morgan's Laws state that (pq) is logically equivalent to pq and that (pq) is logically equivalent to pq.

 

Converse, Inverse and Contrapositive

The converse of pq is qp. The inverse of pq is pq. The contrapositive of pq is qp.

Example: To find the converse, inverse and contrapositive of the statement "If the train is late then the bus is full" we identify the p: "The train is late" and q: "The bus is full" so that the original statement is of the form pq.

From the truth tables below we can see that the converse and the inverse are logically equivalent and that the contrapositive is logically equivalent to the original conditional proposition.

The conditional proposition   Its inverse  
Its converse   Its contrapositive  

Example: Consider the proposition "If you are under five feet tall then I am taller than you". It is logically equivalent to its contrapositive, "If I am not taller than you then you are not under five feet tall". Also its converse, "If I am taller than you then you are under five feet tall", is logically equivalent to its inverse, "If you are not under five feet tall then I am not taller than you".

Next Page: Truth Sets

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