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ZFR
  • Member for 8 years, 4 months
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  • New York, NY, USA
21 votes
3 answers
1k views

Sum of series $\sum \limits_{k=1}^{\infty}\frac{\sin^3 3^k}{3^k}$

18 votes
2 answers
9k views

Infinite group has infinitely many subgroups, namely cyclic subgroups.

16 votes
3 answers
2k views

Erroneous proof that derivative of function is always continuous

15 votes
5 answers
3k views

Number which is simultaneously sum of 2 and 3 squares [closed]

15 votes
2 answers
5k views

Every separable metric space has a countable base

15 votes
7 answers
24k views

A finite set is closed

10 votes
2 answers
10k views

Ternary representation of Cantor set

10 votes
2 answers
2k views

Solving $x^2+\frac{81x^2}{(9+x)^2}=40$ [duplicate]

9 votes
3 answers
7k views

Bounded and closed but not compact in rational numbers

9 votes
2 answers
3k views

The equivalence of definitions Riemann integral

8 votes
3 answers
2k views

Monotone Convergence theorem for decreasing sequence

8 votes
1 answer
1k views

Definition of isomorphism from Herstein's book

8 votes
2 answers
516 views

Problem 30 from Shakarchi Stein's book

7 votes
2 answers
687 views

Difference between topologically complete space and complete metric space

7 votes
3 answers
486 views

Theorem 4.22 from baby Rudin

6 votes
1 answer
1k views

Discontinuity of Dirichlet function

6 votes
2 answers
1k views

Every open set in the real line is the countable union of disjoint intervals

6 votes
1 answer
630 views

Dirichlet series

6 votes
1 answer
131 views

Compact set of real numbers with countably many limit points.

6 votes
2 answers
256 views

Proof $\int \limits_{0}^{\infty}\left(\frac{\sin x}{x}\right)^2dx=\frac{\pi}{2}$ by definition

6 votes
2 answers
2k views

Definition of positive measure

6 votes
3 answers
130 views

Solve the equation $\sqrt{1-x}=2x^2-1+2x\sqrt{1-x^2}$

6 votes
1 answer
217 views

$\int \limits_{E}|f|d\mu<\varepsilon$ whenever $\mu(E)<\delta$.

6 votes
3 answers
686 views

Theorem 2.17 from RCA Rudin

6 votes
1 answer
224 views

Theorem 2.22 from RCA Rudin

6 votes
2 answers
759 views

Problem 15 from Herstein's book

6 votes
2 answers
297 views

Pigeonhole principle in finite sequence

6 votes
2 answers
102 views

Evaluate $\sum\limits_{n=1}^{2016}\frac{1}{n!+(n+1)!+(n+2)!}$

6 votes
3 answers
211 views

Coloring of $\mathbb{R}^3$ into 3 colors

6 votes
2 answers
436 views

A convergent sequence has either a maximum or a minimum or both.

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