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44 changes: 22 additions & 22 deletions README.md
Original file line number Diff line number Diff line change
Expand Up @@ -17,63 +17,63 @@ This repository studies a Boolean-indexed polynomial basis in which every basis

## Main result

Let \(N = 2^m\), let \(F\) be an arbitrary field, and index Boolean vectors by
\(y=(y_0,\ldots,y_{m-1}) \in \{0,1\}^m\). Define
Let $N = 2^m$, let $F$ be an arbitrary field, and index Boolean vectors by
$y=(y_0,\ldots,y_{m-1}) \in \{0,1\}^m$. Define

\[
$$
K_y(X)=\prod_{i=0}^{m-1}\left(X^{2^i}+y_i\right).
\]
$$

The paper proves three structural facts.

### 1. Boolean kernel basis

The family

\[
$$
\{K_y : y\in\{0,1\}^m\}
\]
$$

is a basis of \(F[X]_{<N}\), even though every \(K_y\) has degree exactly \(N-1\).
is a basis of $F[X]_{<N}$, even though every $K_y$ has degree exactly $N-1$.

This is deliberately unlike a degree-graded basis: low degree cannot be read off by simply asking which kernel coefficients vanish.

### 2. Exact change of basis

For

\[
$$
U(X)=\sum_y \lambda_y K_y(X)=\sum_a u_a X^{|a|_2},
\]
$$

the monomial coefficients satisfy

\[
$$
u_a=\sum_{y\ge \bar a}\lambda_y.
\]
$$

Thus kernel coordinates and monomial coordinates are related by a Boolean zeta transform composed with the complement permutation. The inverse is Boolean Möbius inversion. Both admit \(O(N\log N)\) butterfly algorithms.
Thus kernel coordinates and monomial coordinates are related by a Boolean zeta transform composed with the complement permutation. The inverse is Boolean Möbius inversion. Both admit $O(N\log N)$ butterfly algorithms.

### 3. Low-degree filtration in kernel coordinates

Split a kernel index as \(y=(x,h)\), where \(x\in\{0,1\}^k\) and
\(h\in\{0,1\}^{q}\) with \(q=m-k\). Then
Split a kernel index as $y=(x,h)$, where $x\in\{0,1\}^k$ and
$h\in\{0,1\}^{q}$ with $q=m-k$. Then

\[
$$
\deg U < 2^k
\quad\Longleftrightarrow\quad
\lambda_{(x,h)}
=
(-1)^{q-\operatorname{wt}(h)}\mu_x
\]
$$

for a unique function \(\mu:\{0,1\}^k\to F\).
for a unique function $\mu:\{0,1\}^k\to F$.

In characteristic two, \(-1=1\), so this becomes constancy across each high-coordinate fiber:
In characteristic two, $-1=1$, so this becomes constancy across each high-coordinate fiber:

\[
$$
\lambda_{(x,h)}=\mu_x.
\]
$$

## Why this artifact exists

Expand Down Expand Up @@ -101,11 +101,11 @@ The result is relevant to later work on FRI-style folding because folding is fun
The Rust artifact contains deterministic executable checks for:

- the exact coefficient formula;
- naive \(O(N^2)\) vs fast \(O(N\log N)\) zeta transforms;
- naive $O(N^2)$ vs fast $O(N\log N)$ zeta transforms;
- naive vs fast Möbius inversion;
- zeta/Möbius round trips;
- both directions of the low-degree filtration theorem over the Goldilocks prime field;
- boundary cases, including \(m=0\), \(k=m\), invalid dimensions, and characteristic two.
- boundary cases, including $m=0$, $k=m$, invalid dimensions, and characteristic two.

The committed tests are cross-checks, not a formal proof of the theorem.

Expand Down
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