Show that if a ú an odd integer, thn a =1 (md 16).

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Author:Erwin Kreyszig
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Number Theory

### Educational Resource: Mathematical Proofs

#### Problem Statement:
- **Objective:** Show that if \( a \) is an odd integer, then \( a^2 \equiv 1 \ (\text{mod} \ 16) \).

This problem requires us to prove that for any odd integer \( a \), the square of \( a \) is congruent to 1 modulo 16. 

This is a very intriguing property of odd integers and congruence relationships that is fundamental in number theory. We can approach this problem using basic properties of odd numbers and modular arithmetic.

#### Steps for the Proof:

1. **Expression for Odd Integers:**
   - An odd integer \( a \) can be expressed in the form \( a = 2k + 1 \), where \( k \) is an integer.
 
2. **Squaring the Odd Integer:**
   - Square \( a \):
     \[
     a^2 = (2k + 1)^2 = 4k^2 + 4k + 1
     \]
     
3. **Modulo 16 Calculation:**
   - Simplify \( 4k^2 + 4k + 1 \) modulo 16:
     \[
     a^2 \equiv 4k^2 + 4k + 1 \ (\text{mod} \ 16)
     \]

4. **Modulo Properties:**
   - Notice that both \( 4k^2 \) and \( 4k \) are multiples of 4.
     \[
     4k^2 \equiv 0 \ (\text{mod} \ 4) \quad \text{and} \quad 4k \equiv 0 \ (\text{mod} \ 4)
     \]

5. **Summing the Components:**
   - Thus, the sum \( 4k^2 + 4k \equiv 0 \ (\text{mod} \ 16) \):
     \[
     a^2 \equiv 0 + 0 + 1 \ (\text{mod} \ 16) \quad \Rightarrow \quad a^2 \equiv 1 \ (\text{mod} \ 16)
     \]

#### Conclusion:
Therefore, we have shown that for any odd integer \( a \), it holds true that \( a^2 \
Transcribed Image Text:### Educational Resource: Mathematical Proofs #### Problem Statement: - **Objective:** Show that if \( a \) is an odd integer, then \( a^2 \equiv 1 \ (\text{mod} \ 16) \). This problem requires us to prove that for any odd integer \( a \), the square of \( a \) is congruent to 1 modulo 16. This is a very intriguing property of odd integers and congruence relationships that is fundamental in number theory. We can approach this problem using basic properties of odd numbers and modular arithmetic. #### Steps for the Proof: 1. **Expression for Odd Integers:** - An odd integer \( a \) can be expressed in the form \( a = 2k + 1 \), where \( k \) is an integer. 2. **Squaring the Odd Integer:** - Square \( a \): \[ a^2 = (2k + 1)^2 = 4k^2 + 4k + 1 \] 3. **Modulo 16 Calculation:** - Simplify \( 4k^2 + 4k + 1 \) modulo 16: \[ a^2 \equiv 4k^2 + 4k + 1 \ (\text{mod} \ 16) \] 4. **Modulo Properties:** - Notice that both \( 4k^2 \) and \( 4k \) are multiples of 4. \[ 4k^2 \equiv 0 \ (\text{mod} \ 4) \quad \text{and} \quad 4k \equiv 0 \ (\text{mod} \ 4) \] 5. **Summing the Components:** - Thus, the sum \( 4k^2 + 4k \equiv 0 \ (\text{mod} \ 16) \): \[ a^2 \equiv 0 + 0 + 1 \ (\text{mod} \ 16) \quad \Rightarrow \quad a^2 \equiv 1 \ (\text{mod} \ 16) \] #### Conclusion: Therefore, we have shown that for any odd integer \( a \), it holds true that \( a^2 \
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