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This question arose in the context of studying reflections.

## Matrix

Let $n$ be a positive integer.

Denote by $B_n$ the matrix of dimensions $ 2^n times left( n+1 right) $ with entries from $ {0,1} $ such that it satisfies the recursive block relation

$$B_n =

left(

begin{array}{c|c}

underline{0}_{left(2^{n-1} times 1right)} & B_{n-1}\

hline

underline{1}_{left(2^{n-1} times 1right)} & B_{n-1}

end{array}

right)

$$

with the condition

$$

B_1 equiv

begin{bmatrix}

0 & 0 \

1 & 0 \

end{bmatrix}

$$

### Matrix examples

For $ n in {2,3,4} $ obtain

$$

B_2 =

begin{bmatrix}

0 & 0 & 0 \

0 & 1 & 0 \

1 & 0 & 0 \

1 & 1 & 0 \

end{bmatrix},

,

B_3 =

begin{bmatrix}

0 & 0 & 0 & 0 \

0 & 0 & 1 & 0 \

0 & 1 & 0 & 0 \

0 & 1 & 1 & 0 \

1 & 0 & 0 & 0 \

1 & 0 & 1 & 0 \

1 & 1 & 0 & 0 \

1 & 1 & 1 & 0 \

end{bmatrix},

,

B_4 =

begin{bmatrix}

0 & 0 & 0 & 0 & 0 \

0 & 0 & 0 & 1 & 0 \

0 & 0 & 1 & 0 & 0 \

0 & 0 & 1 & 1 & 0 \

0 & 1 & 0 & 0 & 0 \

0 & 1 & 0 & 1 & 0 \

0 & 1 & 1 & 0 & 0 \

0 & 1 & 1 & 1 & 0 \

1 & 0 & 0 & 0 & 0 \

1 & 0 & 0 & 1 & 0 \

1 & 0 & 1 & 0 & 0 \

1 & 0 & 1 & 1 & 0 \

1 & 1 & 0 & 0 & 0 \

1 & 1 & 0 & 1 & 0 \

1 & 1 & 1 & 0 & 0 \

1 & 1 & 1 & 1 & 0 \

end{bmatrix}

$$

### Explicit formula for the matrix elements

It’s not hard to show that

$$

left(B_nright)_{i,j} =

begin{cases}

lfloor {i-1 over 2^{n-j}} rfloor pmod{2}, & text{if $1 le j le n$}

\

0, & text{if $j=n+1$}

end{cases}

$$

## Path

A $B_n$-path $P$ is a set of size $2^n$ where each element is an ordered pair, where the first element is a row index of $B_n$, and the second element is a column index of $B_n$, so that each row index of $B_n$ appears exactly **once** in the elements of $P$.

Notice that $P$ has the form

$$

{

left(i_1,j_1right),left(i_2,j_2right), ldots , left(i_{2^n},j_{2^n}right)

}

$$

where the row indices from all the pairs are **pairwise distinct**.

In other words, a $B_n$-path is equivalent to choosing exactly one element from each and every row of $B_n$ in some order.

Obviously $left(B_n right)_{i_{1},j_{1}} = left(B_n right)_{i_{2},j_{2}}$ does **not** imply that $left(i_1,j_1 right) = left(i_2,j_2 right)$.

## Weighted path

A $B_n$-weight $w$ is an $left(n+1right)$-tuple with non-negative integer entries, such that the sum of its entries is equal to $2^n$.

Fix a $B_n$-weight $w equiv left(mu_1, mu_2, ldots , mu_{n+1} right) $, so $mu_j in mathbb{Z}_{ge 0}, , j in {1,2, ldots, n+1 }$ and $sum_{j=1}^{n+1}{mu_j} = 2^n$.

A $B_n$-path with $B_n$-weight $w$, denoted by $P_w$, is a $B_n$-path such that $mu_1$ of its pair elements have column indices which are equal to $1$, $mu_2$ of the remaining pair elements have column indices which are equal to $2$, and so on, until finally the remaining $mu_{n+1}$ pair elements have column indices which are equal to $n+1$.

Notice that if $mu_k = 0$ for some $ k in {1,2,ldots,n+1} $ then $P_w$ does not have an element pair with $k$ as a column index.

Notice that the number of **distinct** $B_n$-paths with a fixed weight $w$ is given by the multinomial coefficient

$$

binom{mu_1+cdots+mu_{n+1}}{mu_1,ldots,mu_{n+1}}=binom{2^n}{mu_1,ldots,mu_{n+1}}

$$

### Weighted path examples

Consider the matrix $B_2$ and the $B_2$-weight $w equiv left(1,2,1 right)$. A $B_2$-path with $B_n$-weight $w$, denoted by $P_w$, can be, for instance, the set

$$

{

left( 1,1right),left( 2,2right),left( 3,2right),left( 4,3right)

}

$$

Graphically, this $B_2$-path looks like the following (in red):

$$

begin{bmatrix}

color{red}{0} & 0 & 0 \

0 & color{red}{1} & 0 \

1 & color{red}{0} & 0 \

1 & 1 & color{red}{0} \

end{bmatrix}

$$

Another possiblity for $P_w$ is the set

$$

{

left( 1,2right),left( 2,3right),left( 3,2right),left( 4,1right)

}

$$

which looks like the following:

$$

begin{bmatrix}

0 & color{red}{0} & 0 \

0 & 1 & color{red}{0} \

1 & color{red}{0} & 0 \

color{red}{1} & 1 & 0 \

end{bmatrix}

$$

Consider the matrix $B_3$ and the $B_3$-weight $w equiv left(2,0,5,1 right)$. A $B_3$-path with $B_n$-weight $w$, denoted by $P_w$ can be, for instance, the set

$$

{

left( 1,1right),left( 2,1right),left( 3,3right),left( 4,3right),left( 5,3right),left( 6,3right),left( 7,3right),left( 8,4right)

}

$$

Graphically, this $B_3$-path looks like the following (in red):

$$

begin{bmatrix}

color{red}{0} & 0 & 0 & 0 \

color{red}{0} & 0 & 1 & 0 \

0 & 1 & color{red}{0} & 0 \

0 & 1 & color{red}{1} & 0 \

1 & 0 & color{red}{0} & 0 \

1 & 0 & color{red}{1} & 0 \

1 & 1 & color{red}{0} & 0 \

1 & 1 & 1 & color{red}{0} \

end{bmatrix}

$$

Another possiblity for $p_w$ is the set

$$

left(

left( 1,4right),left( 2,3right),left( 3,1right),left( 4,3right),left( 5,3right),left( 6,3right),left( 7,3right),left( 8,1right)

right)

$$

which looks like the following:

$$

begin{bmatrix}

0 & 0 & 0 & color{red}{0} \

0 & 0 & color{red}{1} & 0 \

color{red}{0} & 1 & 0 & 0 \

0 & 1 & color{red}{1} & 0 \

1 & 0 & color{red}{0} & 0 \

1 & 0 & color{red}{1} & 0 \

1 & 1 & color{red}{0} & 0 \

color{red}{1} & 1 & 1 & 0 \

end{bmatrix}

$$

## Parity of a path

The parity of a $B_n$-path $P$ is the sum modulo $2$ of the elements of $B_n$ with row-column indices which correspond to the elements of $P$.

Summation modulo 2 is commutative, so the parity of a $B_n$-path $P$ is given by

$$

sum_{i=1}^{2^n}{left( B_nright)_{i,j_i}} pmod 2

$$

where $j_i$ is the column index in the element pair of $P$ with row index $i$.

Notice that when calculatiing this sum we may ignore the elements of $P$ with column index $j_i=n+1$, because the corresponding elements of $B_n$ are all equal to $0$.

### Parity of a path examples

Consider the following $B_2$-path and $B_3$-path and just take the sum of the red colored $0$‘s and $1$‘s modulo 2.

The $B_2$-path described graphically by

$$

begin{bmatrix}

0 & color{red}{0} & 0 \

0 & 1 & color{red}{0} \

color{red}{1} & 0 & 0 \

1 & 1 & color{red}{0} \

end{bmatrix}

$$

has parity equal to $1$.

The $B_3$-path described graphically by

$$

begin{bmatrix}

0 & color{red}{0} & 0 & 0 \

0 & color{red}{0} & 1 & 0 \

0 & 1 & color{red}{0} & 0 \

0 & 1 & color{red}{1} & 0 \

1 & 0 & color{red}{0} & 0 \

1 & 0 & color{red}{1} & 0 \

1 & 1 & color{red}{0} & 0 \

1 & 1 & 1 & color{red}{0} \

end{bmatrix}

$$

has parity equal to $0$.

Consider the matrix $B_n$.

Fix a $B_n$-weight $w equiv left(mu_1, mu_2, ldots,mu_{n+1} right)$, so $mu_j in mathbb{Z}_{ge 0}, , j in {1,2, ldots, n+1}$ and $sum_{j=1}^{n+1}{mu_j} = 2^n$.

- Show that the number of all distinct $B_n$-paths with weight $w$ and parity equal to $0$ is equal to the number of all distinct $B_n$-paths with weight $w$ and parity equal to $1$,
**if and only if** at least one of the entries of the weight $w$ is an **odd** integer.

Now consider a weight with only **even** entries.

Fix a weight $varpi equiv left(2phi_1, 2phi_2, ldots , 2phi_{n+1} right) $, so $phi_j in mathbb{Z}_{ge 0}, , j in {1,2, ldots, n+1 }$ and $sum_{j=1}^{n+1}{phi_j} = 2^{n-1}$.

- Count the number all distinct $B_n$-paths with weight $varpi$ and parity equal to $0$. Count the same for when the parity is equal to $1$.
- Show that the
**difference** between the number of all distinct $B_n$-paths with weight $varpi$ and parity equal to $0$, and the number of all distinct $B_n$-paths with weight $varpi$ and parity equal to $1$, is **invariant** under any permutation of the entries of $varpi$.

I am looking for references to this kind of problems. I’d appreciate to know about equivalent problems which require less setup, perhaps stated as a problem in graph theory. I am also hoping for some input or hints for these problems. Problem 2 seems to be the most difficult.