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coffeemath
  • Member for 6 years, 8 months
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15 votes
2 answers
1k views

$3\times3$ matrix of distinct positive primes with determinant $0$

12 votes
3 answers
5k views

Catalan numbers and triangulations

12 votes
2 answers
489 views

can the product of four positive integers in A.P. be a square?

10 votes
3 answers
1k views

How is this differential equation solved?

9 votes
1 answer
180 views

For which $n$ does $\binom{2n}{n}$ divide $\binom{4n}{n}$?

9 votes
6 answers
2k views

Are there incongruent pythagorean triangles with the same perimeter and same area?

6 votes
1 answer
101 views

Is the repunit $R_{3^k}$ divisible by $3^k$?

6 votes
2 answers
788 views

Solutions to $\varphi(\varphi(a))=\varphi(\varphi(b))$

6 votes
0 answers
71 views

Solutions to $\sigma(a^2)=\sigma(b^2)$ [divisor sum function]

5 votes
0 answers
60 views

Seeking intuition on hat-check probability depending on parity of $n$

4 votes
1 answer
54 views

Is there $n>5$ whose factorial in base $2$ is a block of $1$'s then all $0$s?

3 votes
2 answers
185 views

Characterizing the set of positive integers which cannot be represented as $p+a^2$

3 votes
2 answers
387 views

Permutation count in cycle graphs

3 votes
2 answers
229 views

Make change for $4.10 using 40 coins, pennies nickels dimes quarters

3 votes
3 answers
170 views

About unique factorization of natural numbers into irreducibles

3 votes
1 answer
85 views

Is every even square a value of $\varphi(n)$?

3 votes
1 answer
94 views

Forms of factors of values of $(x^p-1)/(x-1)$?

2 votes
4 answers
238 views

Seek a multiplication formula for $x^2+xy+y^2$

2 votes
1 answer
472 views

Proof of derangement relation $nD_{n-1}+(-1)^n=D_n$

2 votes
1 answer
124 views

Solutions to $\tau(n)=\varphi(n)$

2 votes
1 answer
52 views

Sudoku prime triples

2 votes
1 answer
113 views

sequences of consecutive primitive roots

2 votes
1 answer
63 views

Given a smooth function $f$ for which $\lim_\limits{x\to\pm \infty}f^{(n)}(x)=0.$ Do the derivatives of $f(1/x)$ also tend to $0$ as $x\to0$?

2 votes
1 answer
56 views

Even number not the sum of two base $2$ palindromes

2 votes
0 answers
61 views

Proportion of odd digits in powers of $2$ base ten

2 votes
2 answers
71 views

For $F(n)$ the $n$-th Fibonacci number, is $F(a)F(b)-F(a+1)F(b-1)$ always $\pm F(m)$ for some $m$?

2 votes
0 answers
53 views

Can the (infinite) graph of a tiling of the plane have no two vertices of the same degree?

1 vote
1 answer
102 views

About the Fibonacci "continued fractions"

1 vote
1 answer
164 views

Can the multiplicative group of invertible residue classes mod $105$ be generated by $2$ residue classes?

1 vote
0 answers
52 views

finite groups $G$ such that any two commuting elements of $G$ lie in some cyclic subgroup of $G$