TCSLib

131 Boolean Analysis — Smolensky Algebra

131.1 Overview

This file develops the Smolensky (algebraic) side of the Razborov–Smolensky lower bound for \(\mathrm{AC}^0[p]\) circuits: it packages low-degree polynomial approximation over \(\mathbb {Z}/p\) on the Boolean cube, converts the pointwise circuit-approximation theorem into a single low-degree polynomial by averaging, sets up the field \(\mathbb {F}_{p^{q-1}}\) together with a nontrivial \(q\)-th root of unity \(\omega \), and then works over the root-of-unity cube \(\{ 1,\omega \} ^n\): multilinear representation, the split at degree \(n/2\), and the final Hamming-ball counting obstruction.

131.2 Declarations

Definition 131.1 MOD \(q\) target inside \(\mathbb {Z}/p\)
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For a prime \(q\), the Boolean function \(\mathrm{MOD}_q\) on \(n\) inputs viewed as a function \(\{ 0,1\} ^n \to \mathbb {Z}/p\): the output bit of the gate operation \(\mathrm{modGateOp}\ q\ n\) is cast into \(\mathbb {Z}/p\).

Definition 131.2 Bad input count on the Boolean cube
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For a target \(f : \{ 0,1\} ^n \to \mathbb {Z}/p\) and a polynomial \(P \in (\mathbb {Z}/p)[X_1,\dots ,X_n]\), the number of Boolean inputs \(x\) with \(P(\mathrm{boolInput}(x)) \neq f(x)\).

Definition 131.3 Low-degree bad-count lower bound
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The predicate asserting that every polynomial \(P\) of total degree at most \(d\) has \(E \le \mathrm{badInputCount}(f, P)\), i.e. \(f\) cannot be computed by a degree-\(d\) polynomial on more than \(2^n - E\) points of the Boolean cube.

Lemma 131.4 Averaging over parameters
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Let \(\alpha ,\beta \) be finite types with \(\beta \) nonempty and let \(\mathrm{Fail} : \alpha \to \beta \to \mathrm{Prop}\) be decidable. If for every \(a\) the number of \(b\) with \(\mathrm{Fail}\, a\, b\) satisfies \(\left\lvert \{ b : \mathrm{Fail}\, a\, b\} \right\rvert \cdot C \le B\cdot \left\lvert \beta \right\rvert \), then there is a single \(b\) with \(\left\lvert \{ a : \mathrm{Fail}\, a\, b\} \right\rvert \cdot C \le B\cdot \left\lvert \alpha \right\rvert \).

Theorem 131.5 One polynomial from a pointwise distribution

Let \(P\) be a family of polynomials over \(\mathbb {Z}/p\) indexed by a nonempty finite seed type. If for every Boolean input \(x\) the number of seeds \(s\) with \((P_s)(\mathrm{boolInput}(x)) \neq f(x)\), times \(2^{\ell }\), is at most \(B\cdot \left\lvert \mathrm{Seed}\right\rvert \), then some seed \(s\) satisfies \(\mathrm{badInputCount}(f, P_s)\cdot 2^{\ell } \le B\cdot 2^{n}\).

Theorem 131.6 Single low-degree polynomial for an \(\mathrm{AC}^0[p]\) circuit

Let \(F\) be a feed-forward circuit with a unique output that only uses \(\mathrm{AC}^0[p]\) gate operations. Then there exists a polynomial \(P\) over \(\mathbb {Z}/p\) with \(\deg P \le \mathrm{circuitDegreeBound}\, p\, \ell \, (\mathrm{depth}\, F)\) whose bad-input count against the function computed by \(F\) satisfies \(\mathrm{badInputCount}\cdot 2^{\ell } \le \left\lvert F\right\rvert \cdot 2^{n}\), where \(\left\lvert F\right\rvert \) is the circuit size.

If a feed-forward circuit \(F\) over the \(\mathrm{AC}^0[p]\) gate set computes \(\mathrm{MOD}_q\) on all Boolean inputs, and if \(\mathrm{MOD}_q\) (viewed in \(\mathbb {Z}/p\)) admits the lower bound \(\mathrm{LowDegreeBadCountLB}\) with parameters \((\mathrm{circuitDegreeBound}\, p\, \ell \, (\mathrm{depth}\, F), E)\), then \(E \cdot 2^{\ell } \le \left\lvert F\right\rvert \cdot 2^{n}\).

Same hypotheses as the previous theorem, but with the bad-count bound taken in the relative form \(E = \delta \cdot 2^{n}\); the conclusion is then the clean size bound \(\delta \cdot 2^{\ell } \le \left\lvert F\right\rvert \).

Definition 131.9 Standard field \(\mathbb {F}_{p^{q-1}}\)
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The Galois field \(\mathbb {F}_{p^{q-1}}\), the standard field choice for the \(\mathrm{MOD}_q\) lower bound.

Lemma 131.10 Exponent \(q-1\) is nonzero
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For a prime \(q\) we have \(q - 1 \neq 0\); this is the side condition needed to compute the cardinality of \(\mathbb {F}_{p^{q-1}}\).

Lemma 131.11 Cardinality of \(\mathbb {F}_{p^{q-1}}\)
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For primes \(p\) and \(q\), the field \(\mathrm{ModqField}\, q\) has exactly \(p^{q-1}\) elements.

Theorem 131.12 A unit of order exactly \(q\)

If \(p \neq q\) are primes, then the unit group of \(\mathbb {F}_{p^{q-1}}\) contains an element \(u\) with \(\mathrm{ord}(u) = q\).

Theorem 131.13 Nontrivial \(q\)-th root of unity

If \(p \neq q\) are primes, then there is \(\omega \in \mathbb {F}_{p^{q-1}}\) with \(\omega ^{q} = 1\) and \(\omega \neq 1\).

Definition 131.14 Root-of-unity cube \(\{ 1,\omega \} ^n\)
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The subtype of vectors \(x : \mathrm{Fin}\, n \to K\) such that each coordinate satisfies \(x_i = 1\) or \(x_i = \omega \).

Theorem 131.15 Polynomial representation on the root cube

Every function \(f : \{ 1,\omega \} ^n \to K\) agrees on the cube with the evaluation of some polynomial \(P \in K[X_1,\dots ,X_n]\), i.e. \(P(x) = f(x)\) for all \(x\) in the cube.

Definition 131.16 Squarefree monomial
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For \(s \subseteq \mathrm{Fin}\, n\), the monomial \(\prod _{i \in s} X_i\).

Definition 131.17 Squarefree (multilinear) polynomial from coefficients
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Given coefficients \(c_S\) indexed by subsets \(S \subseteq \mathrm{Fin}\, n\), the polynomial

\[ \sum _{S} c_S \prod _{i \in S} X_i . \]
Theorem 131.18 Evaluation of a squarefree monomial
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For any \(x : \mathrm{Fin}\, n \to K\), the monomial indexed by \(s\) evaluates to \(\prod _{i \in s} x_i\).

Theorem 131.19 Affine expression is coordinatewise inversion
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If \(\omega \neq 0\) and \(x\) lies in \(\{ 1,\omega \} ^n\), then for every coordinate \(i\),

\[ 1 + \omega ^{-1} - \omega ^{-1} x_i = x_i^{-1}. \]
Theorem 131.20 Degree of a squarefree monomial

The total degree of \(\prod _{i \in s} X_i\) is at most \(\left\lvert s\right\rvert \).

Theorem 131.21 Coordinates on the root cube are nonzero
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If \(\omega \neq 0\) and \(x \in \{ 1,\omega \} ^n\), then \(x_i \neq 0\) for every \(i\).

Theorem 131.22 Top monomial times a complement inverse monomial

For \(\omega \neq 0\), \(x \in \{ 1,\omega \} ^n\) and \(s \subseteq \mathrm{Fin}\, n\),

\[ \Bigl(\prod _{i} x_i\Bigr)\cdot \prod _{i \in s^{c}} x_i^{-1} \; =\; \prod _{i \in s} x_i . \]

Let \(\omega \neq 0\) and let \(c\) be a coefficient family. Then there are polynomials \(P_1, P_2\) of total degree at most \(n/2\) such that for every \(x \in \{ 1,\omega \} ^n\),

\[ \Bigl(\textstyle \sum _S c_S \prod _{i \in S} X_i\Bigr)(x) = P_1(x) + \Bigl(\prod _i x_i\Bigr)\cdot P_2\bigl(1 + \omega ^{-1} - \omega ^{-1}x_1,\dots , 1 + \omega ^{-1} - \omega ^{-1}x_n\bigr). \]
Definition 131.24 Affine inverse coordinate polynomial
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The degree-one polynomial \(1 + \omega ^{-1} - \omega ^{-1} X_i\), which on \(\{ 1,\omega \} \) with \(\omega \neq 0\) computes \(x \mapsto x^{-1}\).

Theorem 131.25 Evaluation of the affine inverse polynomial
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For any \(x : \mathrm{Fin}\, n \to K\), the polynomial \(\mathrm{affineInvPoly}\, \omega \, i\) evaluates to \(1 + \omega ^{-1} - \omega ^{-1} x_i\).

Theorem 131.26 Affine inverse polynomial has degree at most one
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The total degree of \(\mathrm{affineInvPoly}\, \omega \, i\) is at most \(1\).

Definition 131.27 Affine-substituted squarefree monomial
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For \(s \subseteq \mathrm{Fin}\, n\), the product \(\prod _{i \in s}(1 + \omega ^{-1} - \omega ^{-1}X_i)\), i.e. the squarefree monomial after the affine inverse substitution.

Theorem 131.28 Evaluation of the affine squarefree monomial
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For any \(x : \mathrm{Fin}\, n \to K\), the affine squarefree monomial indexed by \(s\) evaluates to \(\prod _{i \in s} (1 + \omega ^{-1} - \omega ^{-1} x_i)\).

Theorem 131.29 Degree of the affine squarefree monomial

The total degree of \(\mathrm{affineSquarefreeMonomial}\, \omega \, s\) is at most \(\left\lvert s\right\rvert \).

Let \(\omega \neq 0\) and let \(c\) be a coefficient family. Then there are polynomials \(P_1, R\) of total degree at most \(n/2\) such that for every \(x \in \{ 1,\omega \} ^n\),

\[ \Bigl(\textstyle \sum _S c_S \prod _{i \in S} X_i\Bigr)(x) = P_1(x) + \Bigl(\prod _i x_i\Bigr) R(x), \]

so that the second factor is already evaluated at \(x\) itself rather than at the affine substitution.

Definition 131.31 Bad count of a polynomial on the root cube
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The number of points \(x \in \{ 1,\omega \} ^n\) on which \(P(x) \neq f(x)\).

Definition 131.32 Hamming distance between functions on the root cube
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The number of points \(x \in \{ 1,\omega \} ^n\) on which two functions \(f, g : \{ 1,\omega \} ^n \to K\) differ.

Definition 131.33 Hamming ball on the root cube
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The finite set of functions \(f : \{ 1,\omega \} ^n \to K\) that differ from a given center function on at most \(e\) points of the cube.

Theorem 131.34 Covering by Hamming balls: counting bound
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Let \(\alpha , \beta , \mathrm{Cand}\) be finite and let \(\mathrm{center} : \mathrm{Cand} \to (\alpha \to \beta )\). If every ball \(\{ f : \left\lvert \{ a : \mathrm{center}\, c\, a \neq f(a)\} \right\rvert \le e\} \) has at most \(B\) elements, and every \(f : \alpha \to \beta \) lies in one of these balls, then \(\left\lvert \alpha \to \beta \right\rvert \le \left\lvert \mathrm{Cand}\right\rvert \cdot B\).

Theorem 131.35 Finite counting obstruction on the root cube

Suppose a finite family \((\mathrm{poly}\, c)_{c \in \mathrm{Cand}}\) represents, on the cube, every polynomial of total degree at most \(D\); suppose each Hamming ball of radius \(e\) around such a candidate has at most \(B\) elements; and suppose \(\left\lvert \{ 1,\omega \} ^n \to K\right\rvert \gt \left\lvert \mathrm{Cand}\right\rvert \cdot B\). Then it is not the case that every function on the cube has a degree-\(\le D\) approximant with at most \(e\) bad points.

Theorem 131.36 Approximating the top monomial approximates everything

Assume \(\omega \neq 0\) and that every function on \(\{ 1,\omega \} ^n\) is represented on the cube by a squarefree polynomial. If \(P\) has total degree at most \(d\) and approximates the top monomial \(x \mapsto \prod _i x_i\) with at most \(e\) bad points, then every function \(f\) on the cube has a polynomial \(Q\) with \(\deg Q \le n/2 + d\) and at most \(e\) bad points.

Theorem 131.37 No low-degree approximant to the top monomial

Assume \(\omega \neq 0\), that every function on \(\{ 1,\omega \} ^n\) has a squarefree representative, and the counting hypothesis that not every function on the cube admits a degree-\(\le n/2 + d\) approximant with at most \(e\) bad points. Then no polynomial of total degree at most \(d\) approximates the top monomial \(x \mapsto \prod _i x_i\) within \(e\) bad points.

Theorem 131.38 Top-monomial inapproximability from finite counting

The same conclusion as the previous theorem, with the abstract counting hypothesis replaced by explicit data: a finite candidate family representing all degree-\(\le n/2 + d\) polynomials on the cube, a bound \(B\) on the size of each Hamming ball of radius \(e\), and the strict inequality \(\left\lvert \{ 1,\omega \} ^n \to K\right\rvert \gt \left\lvert \mathrm{Cand}\right\rvert \cdot B\).