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JMoravitz
  • Member for 7 years, 10 months
  • Last seen this week
183 votes
14 answers
16k views

Why $\sqrt{-1 \cdot {-1}} \neq \sqrt{-1}^2$?

  • 2,033
126 votes
13 answers
21k views

Modular exponentiation by hand ($a^b\bmod c$)

  • 46.2k
70 votes
18 answers
18k views

Proof of the hockey stick/Zhu Shijie identity $\sum\limits_{t=0}^n \binom tk = \binom{n+1}{k+1}$

  • 1,540
48 votes
2 answers
3k views

Is a finite group uniquely determined by the orders of its elements?

44 votes
14 answers
88k views

If you draw two cards, what is the probability that the second card is a queen?

37 votes
1 answer
736 views

Does my "Prime Factor Look-and-Say" sequence always end?

  • 471
31 votes
3 answers
3k views

When is $\binom{n}{k}$ divisible by $n$?

  • 2,086
25 votes
5 answers
9k views

Proof writing: how to write a clear induction proof?

8 votes
1 answer
136 views

Game replacing two numbers by mean

  • 2,531
8 votes
1 answer
567 views

Prove ${n \choose k}^2 = \sum_{i=0}^{k}{n \choose i}{n-i \choose k-i}{n-k \choose k-i}$ using a combinatorial argument

5 votes
0 answers
194 views

Minimum number of points chosen from an $N$ by $N$ grid to guarantee a rectangle?

5 votes
1 answer
586 views

Doubt on grouping of terms in a series

  • 975
3 votes
1 answer
122 views

Is it possible to create horizontal lines of arbitrary length in match-three games?

  • 135
3 votes
7 answers
767 views

Alternate ways to prove that $4$ divides $5^n-1$

  • 23.1k
2 votes
2 answers
2k views

Give an algorithm that computes a fair driving schedule for all people in a carpool over $d$ days

  • 61
2 votes
0 answers
2k views

Imaginary sea of Babab