Direct proofs — Proof techniques Theory of Computation

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⭐ What Is a Direct Proof?

A direct proof is a method where you:

  1. Start by assuming the given condition is true (the “if” part).
  2. Use definitions, logical steps, and known facts to move forward.
  3. Eventually reach the conclusion (the “then” part).

It’s like someone tells you:

“If you press the ON button, the machine will start.”

A direct proof is simply pressing the button and watching the machine start — nothing complicated.

⭐ Why Do We Use Direct Proofs in Theory of Computation?

In Theory of Computation, you work with:

  • sets of strings
  • languages
  • automata
  • mathematical properties
  • closure operations

Many of these statements follow the form:
If property X holds, then property Y must follow.

A direct proof helps you show this in a clean and confident way.

⭐ How a Direct Proof Works (Simple Flow)

Here’s the basic flow of a direct proof:

Assume the hypothesis is true  
      ↓  
Use definitions  
      ↓  
Apply logical steps  
      ↓  
Arrive at the conclusion

You walk from point A to point B using only valid steps.
Nothing hidden. Nothing indirect.

⭐ Simple Everyday Example (Before the TOC Example)

Statement:
If a number is even, then its square is even.

Direct Proof:

  • Let the number be n.
  • Since it is even, we can write it as n = 2k.
  • Now square it: n² = (2k)² = 4k².
  • 4k² is obviously even.

Done. Simple.

⭐ Direct Proof in Theory of Computation (Easy Example)

Statement:
If a string belongs to a regular language ( L ), then it must be accepted by some DFA.

Direct Proof:

  1. Start with the assumption:
    A string ( w ) is in the regular language ( L ).
  2. By the definition of a regular language:
    There exists a DFA ( M ) such that ( L = L(M) ).
  3. Since ( w \in L ), and ( L = L(M) ):
    The DFA ( M ) must accept ( w ).
  4. Therefore:
    If ( w \in L ), it is accepted by a DFA.

That’s it — no detours.
You take the meaning of “regular,” apply it, and reach the conclusion.

⭐ Simple Diagram to Show How a Direct Proof Works

Here is an easy visual to understand the idea:

   [Start: Assume Hypothesis is True]
                    │
                    ▼
        [Use Definitions and Known Facts]
                    │
                    ▼
          [Apply Logical Reasoning]
                    │
                    ▼
        [End: Arrive at the Conclusion]

Another small diagram with an analogy:

   (Given Condition) ---> (Logical Steps) ---> (Conclusion)

     A happens                      so                     B happens

A direct proof is basically this straight path.

⭐ Another Computation-Theory Style Example

Statement:
If two regular languages ( L_1 ) and ( L_2 ) are regular, then their union ( L_1 \cup L_2 ) is also regular.

Direct Proof:

  1. Assume ( L_1 ) and ( L_2 ) are regular.
  2. By definition, there must exist DFA machines ( M_1 ) and ( M_2 ) that accept these languages.
  3. We know a DFA can be built that simulates both machines together (product construction).
  4. This new machine accepts exactly the strings in ( L_1 \cup L_2 ).
  5. Therefore, the union is regular.

Again, a clean and simple path.