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220
How to find a general sum formula for the series: 5+55+555+5555+.....?
math.stackexchange.com

71
'Obvious' theorems that are actually false
math.stackexchange.com

66
'Obvious' theorems that are actually false
math.stackexchange.com

57
Accidental expletives in mathematical literature.
math.stackexchange.com

52
How to find the mod of this large number
math.stackexchange.com

51
$\mathbb{R}$ with the lower limit topology is not second-countable
math.stackexchange.com

51
Mathematicians with both “very abstract” and “very applied” achievements
mathoverflow.net

51
Can $18$ consecutive integers be separated into two groups,such that their product is equal?
math.stackexchange.com

43
A simple finite combinatorial sum I found, that seems to work, would have good reasons to work, but I can't find in the literature.
math.stackexchange.com

42
If $\lvert\operatorname{Hom}(H,G_1)\rvert = \lvert\operatorname{Hom}(H,G_2)\rvert$ for any $H$ then $G_1 \cong G_2$
math.stackexchange.com

39
Proofs of AM-GM inequality
math.stackexchange.com

38
Mathematicians with both “very abstract” and “very applied” achievements
mathoverflow.net

35
What is the general opinion on the Generalized Continuum Hypothesis?
mathoverflow.net

33
Can an uncountable group have a countable number of subgroups?
math.stackexchange.com

33
What is the expected value of the largest of the three dice rolls?
math.stackexchange.com

33
Is this graph a planar graph or not?
math.stackexchange.com

30
Theorems in the form of "if and only if" such that the proof of one direction is extremely EASY to prove and the other one is extremely HARD
math.stackexchange.com

30
Given a convergent series of positive rationals, how many subseries can converge to the same number?
math.stackexchange.com

29
Independence intuition
math.stackexchange.com

29
List of generally believed conjectures which cannot all be true
math.stackexchange.com

29
Can a countable group have uncountably many subgroups?
math.stackexchange.com

28
Partitioning the naturals into an infinite number of large sets
math.stackexchange.com

27
Is it possible to define countability without referring the natural numbers?
math.stackexchange.com

27
How big can a separable Hausdorff space be?
math.stackexchange.com

27
What is the highest number of different mates in 1 you can have in one (legal) position?
chess.stackexchange.com

26
Is there a *simple* example showing that uncorrelated random variables need not be independent?
math.stackexchange.com

26
Widely accepted mathematical results that were later shown to be wrong?
mathoverflow.net

24
Cheap proof that the Sorgenfrey line is normal?
math.stackexchange.com

24
What are the odds of hitting exactly 100 rolling a fair die
math.stackexchange.com

24
Connected, locally connected, path-connected but not locally path-connected subspace of the plane
math.stackexchange.com

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