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Joffan's user avatar
Joffan's user avatar
Joffan
  • Member for 9 years, 1 month
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80 votes

Prove that the equation $x^2-y^2 = 2002$ has no integer solution

54 votes
Accepted

How many subsets of set $\{1,2,\ldots,10\}$ contain at least one odd integer?

52 votes
Accepted

Where does the constant increase by 2 of differences between integer square values come from?

51 votes
Accepted

Every point of a grid is colored in blue, red or green. How to prove there is a monochromatic rectangle?

50 votes
Accepted

Is there a combinatorial interpretation of the triangular numbers?

42 votes
Accepted

How many pairs of numbers are there so they are the inverse of each other and they have the same decimal part?

34 votes
Accepted

The sum of the squares is less than or equal to the square of the sums for all $n$

30 votes

How do I find the diameters of the circles in this geometry puzzle?

27 votes
Accepted

Proving that modular inverse only exists when $\gcd(n,x)=1$

25 votes
Accepted

What is the sum of the $81$ products in the $9 \times 9$ multiplication grid?

23 votes

Prove that the expression is a perfect square

22 votes

Which of the following numbers is greater?

20 votes

Elegant Proof of a simple inequality

20 votes
Accepted

Find two numbers whose sum is 20 and LCM is 24

18 votes
Accepted

If $a^2 + p^2 = b^2$ then $2(a+p+1)$ is a perfect square

18 votes

minimum number of steps for knight in chess

17 votes
Accepted

Is there way to know if ODD number can be expressed as sum of two primes?

17 votes
Accepted

Finding consecutive naturals that all fail to have inverses modulo $70$

16 votes

How does atan(1) * 4 equal PI?

15 votes

Multiple-choice: sum of primes below $1000$

12 votes

Congruence Arithmetic Laws, e.g. in divisibility by $7$ test

12 votes

Showing n! is greater than n to the tenth power

11 votes
Accepted

How to compute $\sqrt{1+2015\sqrt{1 + 2018 \cdot 2016}}$

11 votes

Why sum of interior angles in convex polygon is $(n-2)\cdot 180$

11 votes
Accepted

How to get all the factors of a number using its prime factorization?

10 votes

Find all even numbers that can be represented as a difference of squares in only two ways

9 votes

Where does the constant increase by 2 of differences between integer square values come from?

9 votes
Accepted

How to prove that $4a^4-4a^3+5a^2-4a+1\ge 0$ for all real values of $a$?

9 votes

Make a regular dodecagon from a square without losing area nor cutting into too much pieces.

9 votes

If an integer $m$ is odd how can it be proven that its square would be odd?

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