Can You Think Like A Computer?

Logic Level 3

Computers rely upon strict interpretations of logical statements. Can you think like a computer, and identify the statement below that is false ?

Assume that b b and c c are variables which represent non-negative integers.

A) ¬ c : b : ( 2 b = c ) \neg \forall c:\exists b:(2b = c)

B) c : b : ¬ ( 2 b = c ) \forall c : \exists b : \neg (2b = c )

C) ¬ b : c ( 2 b = c ) \neg \exists b: \forall c (2b=c)

D) c : ¬ b : ( 2 b = c ) \forall c : \neg \exists b: (2b = c)

Note:

These symbols have the following meanings:

  • x \forall x is the universal quantifier, which asserts truth for every possible value of x x within its scope. It can be read as "for all x x ".
  • x \exists x is the existential quantifer, which asserts truth for at least one value of x x within its scope. It can be read as "there exists x x ".
  • ¬ x \neg x is the negation symbol. It can be read as "not x x ".
C D A B

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3 solutions

Discussions for this problem are now closed

Trevor B.
Feb 14, 2014

D logically says "For all c c there does not exist a value of b b such that 2 × b = c . " 2\times b=c." It is easy to prove this false by counterexample. If c = 4 , c=4, then a possible value for b b is 2 , 2, which contradicts the logic of the statement. Therefore, D \boxed{\text{D}} is false.

You also need to prove that A), B) and C) are true.

Patrick Engelmann - 6 years, 1 month ago

A) "Not all c c , such that some of b b satisfies 2 b = c 2b = c " means some (or no) c c and some b b makes this true, which is true. ex. 2 ( 2 ) = ( 4 ) 2*(2) = (4) .

B) "All of c c , such that some of b b satisfies 2 b c 2b \neq c " means any b b can make this equation false for any c c , which is true. ex. 2 b 5 2b \neq 5 for any integers b b .

C) "None of b b , such that all of c c satisfies 2 b = c 2b = c " means if c c changes, b b will not stay the same, which is true. (because f ( x ) = 2 b f(x) = 2b is a 1-by-1 function).

D) "All of c c , such that none of b b satisfies 2 b = c 2b = c " means every number c c can't be written in the form of 2 b 2b , which is false, because even number can be written as 2 k , k Z + 2k, \forall k \in Z^{+} \cup { 0 0 }.

Therefore, the answer is D \boxed{D} .~~~

In the third point "C", i think it should be f ( b ) = 2 b f(b)=2b where you say that f ( x ) = 2 b f(x)=2b is a 1-by-1 function.

Prasun Biswas - 7 years, 3 months ago

Oops, my bad. XD

Samuraiwarm Tsunayoshi - 7 years, 3 months ago

Just click 'edit' on upper right corner and change it ;)

Aakarshit Uppal - 7 years, 3 months ago

if c=2 then there exist b=1 such that 2b=c. hence d is false.

Manish Jha - 7 years, 2 months ago
Aakarshit Uppal
Mar 3, 2014

My solution using C++

 #include<iostream.h>
 int main()
 {
 cout<<"All right, I am nuts. Sue me. Here's the solution, though -\n\n"<<endl;
 cout<<"A)\nNot for all (i.e. For some) c, there exists some b";
 cout<<" such that twice of b is c.\nThis is true.\n"<<endl;
 cout<<"B)\nFor all c, there exists some b such that twice of b";
 cout<<" is not c.\nThis is true.\n"<<endl;
 cout<<"C)\nThere does not exist any b such that for all c,";
 cout<<" twice of b comes out to be c.";
 cout<<"\nOR\n";
 cout<<"For any fixed value of b, \'2b = c\' is not true for ALL c.";
 cout<<"\nThis is true\n"<<endl;
 cout<<"D)\nFor all c, there does not exist any b such that twice of b is c.";
 cout<<"\nOR\n";
 cout<<"\'There exists some b such that twice of b is c.\' is false for all c.";
 cout<<"\nThis is FALSE.\n\n"<<endl;
 cout<<"So, answer (false) is D.";
 return(0);
 }

Ask for proof of any of these in the comments, if you want to ; )

Aakarshit Uppal - 7 years, 3 months ago

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