Top network posts

• 48 Formal proof for $(-1) \times (-1) = 1$
• 37 How did Euler prove the Mersenne number $2^{31}-1$ is a prime so early in history?
• 33 Why is every answer of $5^k - 2^k$ divisible by 3?
• 32 If $a^3 =a$ for all $a$ in a ring $R$, then $R$ is commutative.
• 31 Where does the word "torsion" in algebra come from?
• 29 How to prove the divisibility rule for $3\, $ [casting out threes]
• 27 In a group we have $abc=cba$. Is it abelian?
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