Theorem regarding mod properties

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Discussion Overview

The discussion revolves around the properties of modular arithmetic, specifically the theorem that states if \( a \equiv a' \mod c \) and \( b \equiv b' \mod c \), then \( a \pm b \equiv a' \pm b' \mod c \). Participants explore the implications of this theorem and clarify misunderstandings related to congruences and equivalence classes in modular arithmetic.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the application of the theorem, noting confusion when applying it to specific numbers, particularly regarding \( 10 \equiv 10 \mod 7 \) and \( 3 \equiv 10 \mod 7 \).
  • Another participant asserts that \( 10 \equiv 10 \mod 7 \) is correct and emphasizes the importance of using congruence signs instead of equal signs, highlighting that equivalence classes in modulo arithmetic contain infinitely many elements.
  • A participant expresses confusion about the conditions under which numbers can be equal in modular arithmetic, suggesting a misunderstanding of the definition of congruence.
  • Further clarification is provided regarding the definition of congruence, stating that \( x \equiv y \mod n \) means \( n \) divides \( x - y \), and that this does not depend on the size of the numbers involved.
  • Another participant reiterates the need to distinguish between residues and least positive residues, suggesting a return to foundational concepts in modular arithmetic.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the understanding of congruences and the application of the theorem. Some express confusion about the nature of modular equivalence, while others attempt to clarify these concepts without reaching a consensus.

Contextual Notes

Participants demonstrate varying levels of familiarity with modular arithmetic, leading to misunderstandings about congruences and equivalence classes. There is a lack of clarity on the definitions and implications of modular properties, which remains unresolved.

plusunim
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There is a theorem regarding mod properties such that when a=a'(mod
c) and b=b'(mod c) then a*±b=a'±b' (mod c) right?

Now, 5=5 mod7. applying it to the above, 10=10 mod7, which is not
true since 3=10 mod7. Why? I'm confused.

Thanks
 
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No 10 is congruent to 10 mod 7 and it is congruent to 3. You are mistaken only in thinking this is wrong. really you ought to be susing congruent signs, not equal signs. Any equivalence class has infinitely many elements in it in modulo arithmetic.
 
howcome 10=10 mod7? I thought that you can have the same numbers on both sides only when the numbers are less than the mod. Am I wrong again?

-- and by the way, to type fast I use equal signs, I know it's not fully correct, please bear my momentaneous impatience --

Thanks for replying anyway
 
Why would you only be able to have the same numbers on both sides if they were sufficiently small? x=y mod n means exactly that n divides x-y or equivalently that there is an integerk such that x=kn+y and that is all.
 
plusunim said:
howcome 10=10 mod7? I thought that you can have the same numbers on both sides only when the numbers are less than the mod.
As matt has said already, start from the definition of congruence. You are confusing a residue with a least positive residue (commonly referred to as a remainder).

Again, start from the beginning and work your way up.
 
yes I think so. I'll continue reading on. Thanks for the help :)
 

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