Intereting Posts

How do I calculate these sum-of-sum expressions in terms of the generalized harmonic number?
Closed set in $\ell^1$
Probability each of six different numbers will appear at least once?
Confusion of the decidability of $(N,s)$
Proving that given metric space is complete: $X := (0,\infty)$ and $d:=|\ln(x)-\ln(y)|$
Schwarz's lemma $\Rightarrow$ an analytic conformal map UHP$\to$UHP must be an FLT?
Alternative proofs that $A_5$ is simple
A conceptual understanding of transmutations (and bosonizations) of (braided) Hopf algebras
Truncated alternating binomial sum
Expression for the Maurer-Cartan form of a matrix group
Let $\{a_n\}$ be defined as follows: $a_1 = 2$, $a_{n+1}=\frac{1}{3-a_n}$, if $n \geq 1$. Does $\{a_n\}$ converge?
Is the relation $R = \emptyset$ is it reflexive, symmetric and transitive ? Why?
Problem book on differential forms wanted
Center of the Orthogonal Group and Special Orthogonal Group
How do I calculate the probability distribution of the percentage of a binary random variable?

While published sudoku puzzles typically have a unique solution, one can easily conceive of a sudoku puzzle with two solutions. However, is it possible to construct a sudoku puzzle with exactly 3 different solutions?

Inspired by https://puzzling.stackexchange.com/a/6.

- Number of handshakes
- Number of $(0,1)$ $m\times n$ matrices with no empty rows or columns
- How many bracelets can be formed?
- Combinatorics and Inversion Sequences
- Infinite sum involving ascending powers
- Rooks Attacking Every Square on a Chess Board

- How prove this$\frac{1}{P_{0}P_{1}}+\frac{1}{P_{0}P_{2}}+\cdots+\frac{1}{P_{0}P_{n}}<\sqrt{15n}$
- Submodularity of the product of two non-negative, monotone increasing submodular functions
- Nicer expression for the following differential operator
- Expiring coupon collector's problem
- Different approaches to N balls and m boxes problem
- Better than random
- Binomial Sum Related to Fibonacci: $\sum\binom{n-i}j\binom{n-j}i=F_{2n+1}$
- Find number no of ways to fill a grid with balls
- A problem with 26 distinct positive integers
- How do you prove ${n \choose k}$ is maximum when k is $ \lceil \frac n2 \rceil$ or $ \lfloor \frac n2\rfloor $?

You could use this as a model (this works for $4 \times 4$ but needs to be adapted for $9\times 9$):

$\begin{array}{cccc}

1 & 2 & & \\

3 & 4 & 1 & 2 \\

& 1 & & \\

& 3 & & 1 \end{array} $

With solutions

$\begin{array}{cccc}

1 & 2 & 3 & 4 \\

3 & 4 & 1 & 2 \\

2/4 & 1 & 4/2 & 3 \\

4/2 & 3 & 2/4 & 1 \end{array} $

and

$\begin{array}{cccc}

1 & 2 & 4 & 3 \\

3 & 4 & 1 & 2 \\

2 & 1 & 3 & 4 \\

4 & 3 & 2 & 1 \end{array} $

- How do you rearrange equations with dot products in them?
- An unexpected application of non-trivial combinatorics
- Three Circles Meeting at One Point
- Examples where $H\ne \mathrm{Aut}(E/E^H)$
- Are there more rational or irrational numbers?
- For what powers $k$ is the polynomial $n^k-1$ divisible by $(n-1)^2$?
- eigen decomposition of an interesting matrix (general case)
- Should every group be a monoid, or should no group be a monoid?
- Three consecutive sums of two squares
- Evaluation of Integral $\int_{0}^1 \frac{\arctan x }{1+x} dx$
- How to prove an identity (Trigonometry Angles–Pi/13)
- Finding the integral $I=\int_0^1{x^{-2/3}(1-x)^{-1/3}}dx$
- Computing Fourier transform of power law
- How to compute $\int_0^\infty e^{-a(s^2+1/s^2)}\, ds$
- Finding polynomial given the remainders