Is it possible for the DNF and CNF to be the same

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can $A land C$ be both the conjunctive normal form and the disjunctive normal form of




$(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$








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    Can you add your work to show how you simplified the highlighted expression to $Aland C$?
    – amWhy
    Aug 2 at 14:56










  • Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
    – Sudix
    Aug 2 at 16:39














up vote
2
down vote

favorite












For example



can $A land C$ be both the conjunctive normal form and the disjunctive normal form of




$(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$








share|cite|improve this question

















  • 2




    Can you add your work to show how you simplified the highlighted expression to $Aland C$?
    – amWhy
    Aug 2 at 14:56










  • Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
    – Sudix
    Aug 2 at 16:39












up vote
2
down vote

favorite









up vote
2
down vote

favorite











For example



can $A land C$ be both the conjunctive normal form and the disjunctive normal form of




$(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$








share|cite|improve this question













For example



can $A land C$ be both the conjunctive normal form and the disjunctive normal form of




$(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$










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edited Aug 2 at 15:44









Taroccoesbrocco

3,31941330




3,31941330









asked Aug 2 at 14:51









user10168997

434




434







  • 2




    Can you add your work to show how you simplified the highlighted expression to $Aland C$?
    – amWhy
    Aug 2 at 14:56










  • Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
    – Sudix
    Aug 2 at 16:39












  • 2




    Can you add your work to show how you simplified the highlighted expression to $Aland C$?
    – amWhy
    Aug 2 at 14:56










  • Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
    – Sudix
    Aug 2 at 16:39







2




2




Can you add your work to show how you simplified the highlighted expression to $Aland C$?
– amWhy
Aug 2 at 14:56




Can you add your work to show how you simplified the highlighted expression to $Aland C$?
– amWhy
Aug 2 at 14:56












Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
– Sudix
Aug 2 at 16:39




Yes, the formula $A land C$ can both be interpreted as CNF and DNF.
– Sudix
Aug 2 at 16:39










2 Answers
2






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1
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The short answer is: yes!



Indeed, $A land C$ is both a CNF (because it is a conjunction of two disjunctive clauses, each one made of just one literal) and a DNF (with just one conjunctive clause).



To prove that $A land C$ is a CNF and a DNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$, it remains to prove that $A land C$ is logically equivalent to $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. To prove it, I use logical equivalences listed here as rewriting rules (the use of the associativity is left implicit):



beginalign
(A lor (A land B) lor (lnot A land lnot B land lnot C)) land C &equiv (A lor (lnot A land lnot B land lnot C)) land C &textabsortion law \
&equiv (A lor lnot A) land (A lor lnot B) land (A lor lnot C) land C &textdistributivity law \
&equiv (A lor lnot B) land (A lor lnot C) land C &textidentity law \
&equiv (A lor (lnot B land lnot C)) land C &textdistributivity law \
&equiv (Aland C) lor (lnot B land lnot C land C) &textdistributivity law \
&equiv Aland C &textidentity law
endalign
where the first identity law can be applied since $A lor lnot A$ is a tautology, and the second identity law can be applied because $lnot B land lnot C land C$ is a contradiction.




Remark. Pay attention that a CNF of a given formula is not unique, and similarly for DNF. For instance, also $A land C land (B lor lnot B)$ is a CNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. So, properly speaking it is not clear what you refer to when you talk of the CNF (or the DNF) of a given formula.



More interesting remarks about the non-uniqueness of CNF (or DNF) of a given formula are in this question.






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    A conjunctive normal form is a conjunction of a sequence of disjunctions of a sequence of literals or their negations.   Abreviated as: a conjunction of disjunctions of literals or their negations.



    A single literal is a conjunction of a sequence of one literal.   It is also a disjunction of a sequence of one literal.



    Thus $Awedge C$ is a conjunction of two disjunctions of one literal; and also a disjunction of two conjunctions of one literal.   That is it is both a conjunctive normal form and a disjunctive normal form.




    Now, is it a CNF/DNF for $(Avee((Awedge B)vee(neg Awedgeneg Bwedgeneg C)))wedge C$?



    Well, $C$ must be true for the statement to hold.   When we substitue $top$ for $C$, that becomes $(Avee((Awedge B)vee(neg Awedgeneg Bwedge bot)))wedge top$ which simpliefies to $A$.   So $A$ must be true too.   The value of $B$ is irrelevant.



    Therefore $Awedge C$ is indeed a CNF/DNF equivalent to the statement.






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      2 Answers
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      2 Answers
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      The short answer is: yes!



      Indeed, $A land C$ is both a CNF (because it is a conjunction of two disjunctive clauses, each one made of just one literal) and a DNF (with just one conjunctive clause).



      To prove that $A land C$ is a CNF and a DNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$, it remains to prove that $A land C$ is logically equivalent to $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. To prove it, I use logical equivalences listed here as rewriting rules (the use of the associativity is left implicit):



      beginalign
      (A lor (A land B) lor (lnot A land lnot B land lnot C)) land C &equiv (A lor (lnot A land lnot B land lnot C)) land C &textabsortion law \
      &equiv (A lor lnot A) land (A lor lnot B) land (A lor lnot C) land C &textdistributivity law \
      &equiv (A lor lnot B) land (A lor lnot C) land C &textidentity law \
      &equiv (A lor (lnot B land lnot C)) land C &textdistributivity law \
      &equiv (Aland C) lor (lnot B land lnot C land C) &textdistributivity law \
      &equiv Aland C &textidentity law
      endalign
      where the first identity law can be applied since $A lor lnot A$ is a tautology, and the second identity law can be applied because $lnot B land lnot C land C$ is a contradiction.




      Remark. Pay attention that a CNF of a given formula is not unique, and similarly for DNF. For instance, also $A land C land (B lor lnot B)$ is a CNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. So, properly speaking it is not clear what you refer to when you talk of the CNF (or the DNF) of a given formula.



      More interesting remarks about the non-uniqueness of CNF (or DNF) of a given formula are in this question.






      share|cite|improve this answer



























        up vote
        1
        down vote













        The short answer is: yes!



        Indeed, $A land C$ is both a CNF (because it is a conjunction of two disjunctive clauses, each one made of just one literal) and a DNF (with just one conjunctive clause).



        To prove that $A land C$ is a CNF and a DNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$, it remains to prove that $A land C$ is logically equivalent to $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. To prove it, I use logical equivalences listed here as rewriting rules (the use of the associativity is left implicit):



        beginalign
        (A lor (A land B) lor (lnot A land lnot B land lnot C)) land C &equiv (A lor (lnot A land lnot B land lnot C)) land C &textabsortion law \
        &equiv (A lor lnot A) land (A lor lnot B) land (A lor lnot C) land C &textdistributivity law \
        &equiv (A lor lnot B) land (A lor lnot C) land C &textidentity law \
        &equiv (A lor (lnot B land lnot C)) land C &textdistributivity law \
        &equiv (Aland C) lor (lnot B land lnot C land C) &textdistributivity law \
        &equiv Aland C &textidentity law
        endalign
        where the first identity law can be applied since $A lor lnot A$ is a tautology, and the second identity law can be applied because $lnot B land lnot C land C$ is a contradiction.




        Remark. Pay attention that a CNF of a given formula is not unique, and similarly for DNF. For instance, also $A land C land (B lor lnot B)$ is a CNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. So, properly speaking it is not clear what you refer to when you talk of the CNF (or the DNF) of a given formula.



        More interesting remarks about the non-uniqueness of CNF (or DNF) of a given formula are in this question.






        share|cite|improve this answer

























          up vote
          1
          down vote










          up vote
          1
          down vote









          The short answer is: yes!



          Indeed, $A land C$ is both a CNF (because it is a conjunction of two disjunctive clauses, each one made of just one literal) and a DNF (with just one conjunctive clause).



          To prove that $A land C$ is a CNF and a DNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$, it remains to prove that $A land C$ is logically equivalent to $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. To prove it, I use logical equivalences listed here as rewriting rules (the use of the associativity is left implicit):



          beginalign
          (A lor (A land B) lor (lnot A land lnot B land lnot C)) land C &equiv (A lor (lnot A land lnot B land lnot C)) land C &textabsortion law \
          &equiv (A lor lnot A) land (A lor lnot B) land (A lor lnot C) land C &textdistributivity law \
          &equiv (A lor lnot B) land (A lor lnot C) land C &textidentity law \
          &equiv (A lor (lnot B land lnot C)) land C &textdistributivity law \
          &equiv (Aland C) lor (lnot B land lnot C land C) &textdistributivity law \
          &equiv Aland C &textidentity law
          endalign
          where the first identity law can be applied since $A lor lnot A$ is a tautology, and the second identity law can be applied because $lnot B land lnot C land C$ is a contradiction.




          Remark. Pay attention that a CNF of a given formula is not unique, and similarly for DNF. For instance, also $A land C land (B lor lnot B)$ is a CNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. So, properly speaking it is not clear what you refer to when you talk of the CNF (or the DNF) of a given formula.



          More interesting remarks about the non-uniqueness of CNF (or DNF) of a given formula are in this question.






          share|cite|improve this answer















          The short answer is: yes!



          Indeed, $A land C$ is both a CNF (because it is a conjunction of two disjunctive clauses, each one made of just one literal) and a DNF (with just one conjunctive clause).



          To prove that $A land C$ is a CNF and a DNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$, it remains to prove that $A land C$ is logically equivalent to $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. To prove it, I use logical equivalences listed here as rewriting rules (the use of the associativity is left implicit):



          beginalign
          (A lor (A land B) lor (lnot A land lnot B land lnot C)) land C &equiv (A lor (lnot A land lnot B land lnot C)) land C &textabsortion law \
          &equiv (A lor lnot A) land (A lor lnot B) land (A lor lnot C) land C &textdistributivity law \
          &equiv (A lor lnot B) land (A lor lnot C) land C &textidentity law \
          &equiv (A lor (lnot B land lnot C)) land C &textdistributivity law \
          &equiv (Aland C) lor (lnot B land lnot C land C) &textdistributivity law \
          &equiv Aland C &textidentity law
          endalign
          where the first identity law can be applied since $A lor lnot A$ is a tautology, and the second identity law can be applied because $lnot B land lnot C land C$ is a contradiction.




          Remark. Pay attention that a CNF of a given formula is not unique, and similarly for DNF. For instance, also $A land C land (B lor lnot B)$ is a CNF of $(A lor ((A land B) lor (lnot A land lnot B land lnot C))) land C$. So, properly speaking it is not clear what you refer to when you talk of the CNF (or the DNF) of a given formula.



          More interesting remarks about the non-uniqueness of CNF (or DNF) of a given formula are in this question.







          share|cite|improve this answer















          share|cite|improve this answer



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          edited Aug 3 at 6:43


























          answered Aug 2 at 16:54









          Taroccoesbrocco

          3,31941330




          3,31941330




















              up vote
              0
              down vote













              A conjunctive normal form is a conjunction of a sequence of disjunctions of a sequence of literals or their negations.   Abreviated as: a conjunction of disjunctions of literals or their negations.



              A single literal is a conjunction of a sequence of one literal.   It is also a disjunction of a sequence of one literal.



              Thus $Awedge C$ is a conjunction of two disjunctions of one literal; and also a disjunction of two conjunctions of one literal.   That is it is both a conjunctive normal form and a disjunctive normal form.




              Now, is it a CNF/DNF for $(Avee((Awedge B)vee(neg Awedgeneg Bwedgeneg C)))wedge C$?



              Well, $C$ must be true for the statement to hold.   When we substitue $top$ for $C$, that becomes $(Avee((Awedge B)vee(neg Awedgeneg Bwedge bot)))wedge top$ which simpliefies to $A$.   So $A$ must be true too.   The value of $B$ is irrelevant.



              Therefore $Awedge C$ is indeed a CNF/DNF equivalent to the statement.






              share|cite|improve this answer

























                up vote
                0
                down vote













                A conjunctive normal form is a conjunction of a sequence of disjunctions of a sequence of literals or their negations.   Abreviated as: a conjunction of disjunctions of literals or their negations.



                A single literal is a conjunction of a sequence of one literal.   It is also a disjunction of a sequence of one literal.



                Thus $Awedge C$ is a conjunction of two disjunctions of one literal; and also a disjunction of two conjunctions of one literal.   That is it is both a conjunctive normal form and a disjunctive normal form.




                Now, is it a CNF/DNF for $(Avee((Awedge B)vee(neg Awedgeneg Bwedgeneg C)))wedge C$?



                Well, $C$ must be true for the statement to hold.   When we substitue $top$ for $C$, that becomes $(Avee((Awedge B)vee(neg Awedgeneg Bwedge bot)))wedge top$ which simpliefies to $A$.   So $A$ must be true too.   The value of $B$ is irrelevant.



                Therefore $Awedge C$ is indeed a CNF/DNF equivalent to the statement.






                share|cite|improve this answer























                  up vote
                  0
                  down vote










                  up vote
                  0
                  down vote









                  A conjunctive normal form is a conjunction of a sequence of disjunctions of a sequence of literals or their negations.   Abreviated as: a conjunction of disjunctions of literals or their negations.



                  A single literal is a conjunction of a sequence of one literal.   It is also a disjunction of a sequence of one literal.



                  Thus $Awedge C$ is a conjunction of two disjunctions of one literal; and also a disjunction of two conjunctions of one literal.   That is it is both a conjunctive normal form and a disjunctive normal form.




                  Now, is it a CNF/DNF for $(Avee((Awedge B)vee(neg Awedgeneg Bwedgeneg C)))wedge C$?



                  Well, $C$ must be true for the statement to hold.   When we substitue $top$ for $C$, that becomes $(Avee((Awedge B)vee(neg Awedgeneg Bwedge bot)))wedge top$ which simpliefies to $A$.   So $A$ must be true too.   The value of $B$ is irrelevant.



                  Therefore $Awedge C$ is indeed a CNF/DNF equivalent to the statement.






                  share|cite|improve this answer













                  A conjunctive normal form is a conjunction of a sequence of disjunctions of a sequence of literals or their negations.   Abreviated as: a conjunction of disjunctions of literals or their negations.



                  A single literal is a conjunction of a sequence of one literal.   It is also a disjunction of a sequence of one literal.



                  Thus $Awedge C$ is a conjunction of two disjunctions of one literal; and also a disjunction of two conjunctions of one literal.   That is it is both a conjunctive normal form and a disjunctive normal form.




                  Now, is it a CNF/DNF for $(Avee((Awedge B)vee(neg Awedgeneg Bwedgeneg C)))wedge C$?



                  Well, $C$ must be true for the statement to hold.   When we substitue $top$ for $C$, that becomes $(Avee((Awedge B)vee(neg Awedgeneg Bwedge bot)))wedge top$ which simpliefies to $A$.   So $A$ must be true too.   The value of $B$ is irrelevant.



                  Therefore $Awedge C$ is indeed a CNF/DNF equivalent to the statement.







                  share|cite|improve this answer













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                  answered Aug 2 at 23:39









                  Graham Kemp

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