The following logical equivalence shows us that every WFF that uses can be written with and instead.
Let and be WFFs. Then
Here is the truth table that proves this result:
T | T | T | T |
T | F | F | F |
F | T | T | T |
F | F | T | T |
This equivalence is commonly used when proving a statement like “ implies .” Proofs of statements in this form are often carried out by assuming that is true and then deducing that is also true. Why is that sufficient to prove ?
Suppose that if is true, so is . If is false then is true, so is true no matter what the statements and were. On the other hand if is true we know is true as well, so is true in that case too. So regardless of the truth value of , the formula is true. Because this is logically equivalent to , we’re done.
The contrapositive of an implication is by definition . For example, the contrapositive of “if it’s Monday, then it’s raining” is “if it’s not raining, then it’s not Monday.” We are going to use the logical equivalence of the previous section to show that an implication is logically equivalent to its contrapositive.
Let and be WFFs. Then
Again this is very useful as a proof technique. If you want to prove , it is logically equivalent to prove the contrapositive , and this is sometimes easier. An example is
is an irrational number implies is an irrational number.
This statement is true, but the contrapositive “ is rational implies is rational” is easier to prove because being rational actually tells you something specific (that for some whole numbers and ) which you can use to make the proof work. There are further examples given in this blog post by Timothy Gowers.
Don’t confuse the contrapositive of an implies statement with its converse. The converse of is defined to be , and these two are not in general logically equivalent. (You should think of a truth assignment to show that and are not logically equivalent.)