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In everyday life we often combine propositions to form more complex propositions without paying much attention to them. For example combining "Grass is green", and "The sun is red" we say something like "Grass is green and the sun is red", "If the sun is red, grass is green", "The sun is red and the grass is not green" etc. Here "Grass is green", and "The sun is red" are propositions, and form them using connectives "and", "if... then ..." and "not" a little more complex propositions are formed. These new propositions can in turn be combined with other propositions to construct more complex propositions. They then can be combined to form even more complex propositions. This process of obtaining more and more complex propositions can be described more generally as follows:
Let X and Y represent arbitrary propositions. Then [¬X], [X⋀Y], [X⋁Y], [X→Y], and [X↔Y] are propositions.
Note that X and Y here represent an arbitrary proposition. This is actually a part of more rigorous definition of proposition which we see later.
Example : [ P → [Q ⋁ R] ]is a proposition and it is obtained by first constructing [Q ⋁ R] by applying [X ⋁ Y]to propositions Q and R considering them as X and Y, respectively, then by applying [ X→Y ] to the two propositions P and [Q ⋁ R]considering them as X and Y, respectively.
Note: Rigorously speaking X and Y above are place holders for propositions, and so they are not exactly a proposition. They are called a propositional variable, and propositions formed from them using connectives are called a propositional form. However, we are not going to distinguish them here, and both specific propositions such as "2 is greater than 1" and propositional forms such as (P ⋁Q) are going to be called a proposition.
For the proposition P→Q, the proposition Q→P is called its converse, and the proposition ¬ Q→ ¬ P is called its contrapositive.
For example for the proposition "If it rains, then I get wet",
Converse: If I get wet, then it rains.
Contrapositive: If I don't get wet, then it does not rain.
The converse of a proposition is not necessarily logically equivalent to it, that is they may or may not take the same truth value at the same time.
On the other hand, the contrapositive of a proposition is always logically equivalent to the proposition. That is, they take the same truth value regardless of the values of their constituent variables. Therefore, "If it rains, then I get wet." and "If I don't get wet, then it does not rain." are logically equivalent. If one is true then the other is also true, and vice versa.
If-then statements appear in various forms in practice. The following list presents some of the variations. These are all logically equivalent, that is as far as true or false of statement is concerned there is no difference between them. Thus if one is true then all the others are also true, and if one is false all the others are false.
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