Proving Theorems Not in "If-Then" Form

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Discussion Overview

The discussion revolves around the challenges of proving theorems that are not expressed in the standard "if p, then q" format. Participants explore various proof techniques applicable to statements presented as facts or in other forms, questioning the categorization of proofs based on their structure.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions how to prove theorems stated simply as facts, suggesting that direct proof and contrapositive methods may not apply.
  • Another participant argues that all statements can be framed as "if p, then q," implying that the proposed categories for proofs may not exist.
  • A different viewpoint suggests that what constitutes a fact in mathematics relies on axioms and definitions, highlighting that definitions can alter the status of a statement as a fact.
  • Some participants note that different methods of proof may depend on the form of the proposition, indicating that the approach to proving a statement is influenced by its structure.
  • There is a discussion about the organization of a textbook that separates conditional and non-conditional statements, with participants questioning whether this categorization is appropriate.
  • One participant emphasizes that most proofs involve a combination of techniques, suggesting that the complexity of proofs cannot be reduced to a single method.
  • Another participant points out that "non-conditional statements" often include equivalences, which can be expressed as conditional statements, further complicating the categorization.

Areas of Agreement / Disagreement

Participants express differing views on the categorization of proofs and the applicability of proof techniques based on the structure of statements. There is no consensus on whether the organization of proofs into conditional and non-conditional categories is valid.

Contextual Notes

Some statements depend on specific definitions, and the discussion acknowledges that the interpretation of facts can vary based on these definitions. The complexity of proofs is noted, with an emphasis on the interplay of multiple techniques.

Mr Davis 97
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My textbook goes into depth about proof techniques and about how to go about proving theorems. However, the author only really focuses on theorems that are stated in the form "if p, then q." I know that a great many theorems have this logical structure, so it is good to know how to prove them, using direct, contrapositive, and contradiction techniques. However, what if a theorem does not have this "if p, then q" structure? What if it is just stated as a fact, p? How are these types of statements proved in general? We can't use a direct proof, because we don't have a hypothesis, and we can't use contrapositive because it is not in the conditional form. Can we only prove it using definitions and/or contradiction?
 
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"if p, then q" is a fact "r", and every such statement can be seen as "if 1=1, then r". The categories you want to make do not exist.
 
Facts in mathematics are axioms. It's only important that a set of axioms is without a contradiction.
What you might have in mind to be a fact very likely depends on some given definitions. E.g. a statement "2 is an even number." seems to be a fact, but it depends on the definition of even numbers and could be stated: "Even numbers are divisible by 2. Then 2 is an even number." If you define even numbers as those, which are not divisible by 2, then 2 is not an even number. Both is possible. The fact that it would be unusual doesn't make it wrong as a logic statement.
 
mfb said:
"if p, then q" is a fact "r", and every such statement can be seen as "if 1=1, then r". The categories you want to make do not exist.
Well there are different methods of proof depending on what form the proposition is in, right?
 
Check out the first 5 chapters of "an interactive introduction to mathematical analysis" by jonathan Lewin
 
Mr Davis 97 said:
Well there are different methods of proof depending on what form the proposition is in, right?
The easiest way to prove something (there are always many options) depends only on what you want to prove, not on the way it is written down.
 
mfb said:
The easiest way to prove something (there are always many options) depends only on what you want to prove, not on the way it is written down.

I agree with your statement, but just as an example of where I might have the wrong impression is with "Book of Proof," whose table of contents are here.
  • Part I: Fundamentals

    1. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Sets.pdf

    2. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Logic.pdf

    3. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Counting.pdf
  • Part II: How to Prove Conditional Statements

    4. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Direct.pdf

    5. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Contrapositive.pdf

    6. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Contradict.pdf
  • Part III: More on Proof

    7. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Other.pdf


    8. http://www.people.vcu.edu/%7Erhammack/BookOfProof/SetProofs.pdf

    9. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Disproof.pdf

    10. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Induction.pdf
  • Part IV: Relations, Functions and Cardinality

    11. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Relations.pdf

    12. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Functions.pdf

    13. http://www.people.vcu.edu/%7Erhammack/BookOfProof/Cardinality.pdf

As you can see, it has one section "How to Prove Conditional Statements," and another "Proving Non-Conditional Statements," so it makes it seem as though proofs can be categorized by how they are written. Is the author wrong to organize the book in this way?
 
Mr Davis 97 said:
As you can see, it has one section "How to Prove Conditional Statements," and another "Proving Non-Conditional Statements," so it makes it seem as though proofs can be categorized by how they are written. Is the author wrong to organize the book in this way?
This is obviously a book about proofs. Therefore the author needed to partition it into chapters.

Most proofs are build by a combination of several techniques. E.g. ##A \Rightarrow B \Rightarrow C## may be shown by
"Given ##A##, we assume ##\lnot B## which leads to a contradiction, next we can directly conclude ##B \Rightarrow C##"
which is a frequently used pattern. There are proofs that fill entire books. Can you imagine they were done by a single technique?

What the author calls "Non-Conditional Statements" starts with "if-and-only-if" (iff) statements, i.e. equivalences. But these are simply two conditional statements combined:
"##A \Longleftrightarrow B\;##" is identical to "##A \Longrightarrow B \wedge B \Longrightarrow A\;##".
By the way, this does not mean that either were true (and therefore both were true). They both maybe false:
"##5## is divisible by ##2##, if and only if ##3## is divisible by ##2##" is a true statement although neither ##5## nor ##3## is actually divisible by ##2##.
 
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