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Hurkyl

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56
Find a real function, $f$, s.t. $f(f(x)) = -x$.
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28
Are all infinities equal?
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26
Find all polynomials $P$ such that $P(x^2+1)=P(x)^2+1$
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24
I have learned that 1/0 is infinity, why isn't it minus infinity?
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23
Seems that I just proved $2=4$.
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21
Is it wrong to tell children that 1/0 = NaN is incorrect, and should be ∞?
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21
Is there a proof of Gödel's Incompleteness Theorem without self-referential statements?
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19
An example for a calculation where imaginary numbers are used but don't occur in the question or the solution.
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17
Are these exactly the abelian groups?
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16
Why is $\log_{-2}{4}$ complex?
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16
Why is 'abuse of notation' tolerated?
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16
A limit wrong using Wolfram Alpha
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14
How can I algebraically prove that $2^n - 1$ is not always prime?
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13
What is $dx$ in integration?
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13
Is there really such a thing as a tangent line?
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12
Neatest proof that set of finite subsets is countable?
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11
Prove: $\frac{n^5}5 + \frac{n^4}2 + \frac{n^3}3 - \frac n {30}$ is an integer for $n \ge 0$
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10
How to show that $\mathrm{SL}(2,\mathbb Z) = \langle A, B\rangle$?
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10
What are rational integer coefficients?
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10
How to avoid perceived circularity when defining a formal language?
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10
It's in my hands to have a surjective function
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10
A sentence false in a field of characteristic $0$ but true in all fields of positive characteristic?
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9
Is 2048 the highest power of 2 with all even digits (base ten)?
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9
Divergent series and $p$-adics
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9
Does $i^i$ and $i^{1\over e}$ have more than one root in $[0, 2 \pi]$
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9
How to check if a point is inside a rectangle?
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9
Irreducible polynomial means no roots?
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9
Is a counterexample considered a rigorous proof that a property is not true?
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9
Ideals of $\mathbb{Z}[X]$
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9
p-adic numbers and $\mathbb{F}_p$
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