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Implication ñ Conditional Statement
p → q (p implies q) (if p then q) is the proposition that is false when p is true and q is false and true otherwise.
Equivalent to ìnot p or qî
Ex.
If I am elected then I will lower the taxes
If you get 100% on the final then you will get an A
p: I am elected
q: I will lower the taxes
Think of it as a contract, obligation or pledge
The contract is breached (implication is false) only if the p is true and q is false. Otherwise the contract (the pledge) is valid (true)
p → q (p implies q)
p: hypothesis, premise, antecedent
q: conclusion, consequence
p and q do not have to be related
If 2+2 = 4 then Albany is the capitol of N.Y. (T) If 2+2 = 4 then Peapack is the capitol of N.J. (F) If 2+2 = 5 then there is a state with only one neighbor (T) If 2+2 = 5 then you are the pope Also (T)
p → q (p implies q)
p is sufficient for q
a sufficient condition for q is p
q is necessary for p
a necessary condition for p is q
Ex. If you get 100% on the final you will get an A You get 100% on the final is sufficient to get an A A sufficient condition to get an A is to get 100% on the final
You get an A is necessary for you to get 100% on the final (but not sufficient) A necessary condition for you get 100% is you get an A
(imagine that you know your letter grade and you are trying to guess your grade in the final: If you didnít get A then for sure you didnít get 100% If you get A then you may or may not get 100% )
p → q (p implies q)
p is sufficient for q
a sufficient condition for q is p
q is necessary for p
a necessary condition for p is q
If you show up on Monday you will get the job
You show up on Monday is sufficient for you to get the job A sufficient condition for you to get the job is to show up on Monday You get the job is necessary for you have shown up on Monday. A necessary condition for you have shown up on Monday is you got the job.
ï Contrapositive
The contrapositive of ìif p then qî is
ìif not q then not pî
The contrapositive of
p → q is ¬ q → ¬ p
A conditional statement is logically equivalent to its contrapositive.
Proof by truth table.
Ex. If today is Easter then tomorrow is Monday
Contrapositive:
If tomorrow is not Monday then today is not Easter
ï If p then q (p → q )
ï The converse is ì If q then pî
q → p
ï The inverse is ìIf not p then not qî
¬ p → ¬ q
ï The contrapositive ìif not q then not pî
¬ q → ¬ p Ex. If today is Easter then tomorrow is Monday
Converse: If tomorrow is Monday then today is Easter Inverse: If today is not Easter then tomorrow is not Monday
Conditional statement is not equiv. to its converse Conditional statement is not equiv. to its inverse
The converse and inverse are logically equivalent.
ï Your guarantee is good only if you bought your CD player less than 90 days ago
Which statement is equivalent?
- if you bought your CD player less than 90 days ago then your guarantee is good
- If your guarantee is good then you have bought your CD player less than 90 days ago
- You bought your CD player less than 90 days ago is sufficient for the guarantee to be good.
- You bought your CD player less than 90 days ago is necessary for the guarantee to be good.
1 and 3 are not equivalent .. Why?
The biconditional p ↔ q is the proposition
that is true when p and q have the same truth values, and is false otherwise.
Biconditional is equivalent to two way implication:
p ↔ q ≡ p → q ∧ q → p
p → q : if p then q : p only if q
q → p : if q then p : p if q
p ↔ q : p if and only if q
or,
p ↔ q : p iff q
: q iff p
F F T
F T F
T F F
T T T
p q p ↔ q
Ex: This program is correct if, and only if, it produces the correct answer for all possible sets of input data.
This is equivalent to: If this program is correct, then it produces the correct answer for all possible sets of input data and if it produces the correct answer for all possible sets of input data then this program is correct.
This is equivalent to: This program is correct is necessary and sufficient condition for it to produce correct answer for all possible sets of input data.
This is also equivalent to: This program produces the correct answer for all possible sets of input data is necessary and sufficient condition for the program to be correct
