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18
If $\sum_{n=1}^{\infty}{a_{[f(n)]}}$ converges, then $\sum_{n=1}^{\infty}{\frac{a_n}{f(n)}}$ converges.
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11
Are there sequence $(u_n)\subset \mathbb R^+$ s.t. $\sum_{n=1}^\infty u_n<\infty $ but $nu_n\not\to 0$?
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11
What is the last non-zero digit of $(\dots((2018\underset{! \text{ occurs }1009\text{ times}}{\underbrace{!)!)!\dots)!}}$?
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11
What kind of mathematical rule was broken in here?
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10
how do I compare those two logs?
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10
Australian Math Competition Problem
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9
Each group of order 8 has a subgroup of order 2 and a subgroup of order 4.
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8
$n$ lines in a plane, proper coloring of intersection points with just 3 colors
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8
Contest math problem about crossing out numbers in the table
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8
Why $P(x) = 0$ is equivalent to $\dfrac{P(x)}{P'(x)} = 0$ for every polynomial $P$?
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7
How can a finite set not have a maximum or minimum?
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7
Integers $1,2,...,n$ are placed in a way that each value is either bigger or smaller than all preceding values. In how many ways this can be done?
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6
Why there is no formula log(a) * log(b) = (something)?
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6
Graph with same number of edges and vertices is a cycle?
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6
Notice that $\sqrt{51}\approx 7+\frac{\sqrt{2}}{10}$
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6
If $G$ is a non cyclic group of order 27 find the number of elements $x\in G$ such that $x^9 = e$.
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6
Given a continuous function. Show that there exists x,y with x is not y so that f(x)=f(y)
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6
Is it possible to compute $-\log\left({\sqrt{1.8\times 10^{-5}\times 0.1}}\right)$ without a calculator?
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6
Suppose $|G| < \infty$ and $H \leq G$ such that the product of any two left cosets has the same cardinality as $H$. Is $H$ is normal?
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6
If an n x n matix A is not invertible, then is 0 an eigenvalue of A?
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6
Given $8\sqrt{p} = q\sqrt{80}$, where $p$ is prime, is the solution unique?
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6
Finding the smallest possible $n$ such that $S_{n}$ has an element of a given order.
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5
Bipartite subgraphs with lots of edges
mathoverflow.net

5
Every natural number $n$ can be written as $n=s-t$ with $\omega(s)=\omega(t)$
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5
What is the asymptotic form for the sum of the reciprocals of the first $n$ primes?
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5
Prove that every undirected graph has some orientation that is a Directed Acyclic Graph.
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5
Prove that an integer $a$ which satisfies the equation $a^2=1+2b^2$ for integer $b$ is of the form $a=c^2\pm1$ for some integer $c$
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5
very hard proof on binary tree on number of child's
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5
Numbers of the form $\sum_{n=0}^{N} 2^{a_n} 3^n$
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5
If $ab^p = b^qa$ then show that $b=e$ for a group $G$ with order $pq, p$ and $q$ primes.
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