Blueprint of Groebner Basis Theory in LEAN

2 Lemmas

Lemma 5 finset_subset_preimage_of_finite_image

Let \(f: \alpha \to \beta \) be a function and \(s \subseteq \alpha \) a subset with finite image \(f(s)\). Then there exists a finite subset \(s' \subseteq _{\text{fin}} s\) such that:

  • \(s' \subseteq s\) (subset relation)

  • \(f(s') = f(s)\) (image equality)

  • \(|s'| = |f(s)|\) (cardinality preservation)

Proof
Lemma 6 subset_finite_subset_subset_span
Proof
Theorem 7 Submodule.fg_span_iff_fg_span_finset_subset
Proof
Lemma 8 zero_le
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a Partially Ordered Set, a ≥0

Proof
Lemma 9 degree_mem_support_iff
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Proof
Lemma 10 IsRemainder_def’
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Let \(p \in R[\mathbf{X}]\), \(G'' \subseteq R[\mathbf{X}]\) be a set of polynomials, and \(r \in R[\mathbf{X}]\). Then \(r\) is a remainder of \(p\) modulo \(G''\) with respect to monomial order \(m\) if and only if there exists a finite linear combination from \(G''\) such that:

  1. The support of the combination is contained in \(G''\)

  2. \(p\) decomposes as the sum of this combination and \(r\)

  3. For each \(g' \in G''\), the degree of \(g' \cdot (coefficient\ of\ g')\) is bounded by \(\deg _m(p)\)

  4. No term of \(r\) is divisible by any leading term of non-zero elements in \(G''\)

Proof
Lemma 11 IsRemainder_def’’

Let \(p, r \in k[x_i : i \in \sigma ]\), and let \(G' \subseteq k[x_i : i \in \sigma ]\) be a finite set. We say that \(r\) is a generalized remainder of \(p\) upon division by \(G'\) if the following two conditions hold:

  1. For every nonzero \(g \in G'\) and every monomial \(x^s \in \operatorname {supp}(r)\), there exists some component \(j \in \sigma \) such that

    \[ \operatorname {multideg}(g)_j {\gt} s_j. \]
  2. There exists a function \(q : G' \to k[x_i : i \in \sigma ]\) such that:

- For every $g \in G'$,
      $$
      \operatorname{multideg}''(q(g)g) \leq \operatorname{multideg}''(p);
      $$
- The decomposition holds:
      $$
      p = \sum_{g \in G'} q(g)g + r.
      $$
Proof
Lemma 12 lm_eq_zero_iff
#

Let \(p \in R[\mathbf{X}]\) be a multivariate polynomial. Then the leading term of \(p\) vanishes with respect to monomial order \(m\) if and only if \(p\) is the zero polynomial:

\[ \operatorname{LT}_m(p) = 0 \iff p = 0 \]
Proof
Lemma 13 leadingTerm_image_sdiff_singleton_zero

For any set of polynomials \(G'' \subseteq R[\mathbf{X}]\) and monomial order \(m\), the image of leading terms on the nonzero elements of \(G''\) equals the image on all elements minus zero:

\[ \operatorname{LT}_m(G'' \setminus \{ 0\} ) = \operatorname{LT}_m(G'') \setminus \{ 0\} \]
Proof
Lemma 14 leadingTerm_image_insert_zero
Proof
Lemma 15 isRemainder_of_insert_zero_iff_isRemainder

Let \(p \in R[\mathbf{X}]\) be a polynomial, \(G'' \subseteq R[\mathbf{X}]\) a set of polynomials, and \(r \in R[\mathbf{X}]\) a remainder. Then the remainder property is invariant under inserting the zero polynomial:

\[ \mathsf{IsRemainder}_m\, p\, (G'' \cup \{ 0\} )\, r \iff \mathsf{IsRemainder}_m\, p\, G''\, r \]
Proof
Lemma 16 isRemainder_sdiff_singleton_zero_iff_isRemainder

Let \(p \in R[\mathbf{X}]\) be a polynomial, \(G'' \subseteq R[\mathbf{X}]\) a set of polynomials, and \(r \in R[\mathbf{X}]\) a remainder. Then the remainder property is invariant under removal of the zero polynomial:

\[ \mathsf{IsRemainder}_m\, p\, (G'' \setminus \{ 0\} )\, r \iff \mathsf{IsRemainder}_m\, p\, G''\, r \]
Proof
Lemma 17 sPolynomial_antisymm
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the S-polynomial of \(f\) and \(g\) is antisymmetric:

\[ \operatorname{Sph}{f}{g} = -\operatorname{Sph}{g}{f} \]
Proof
Lemma 18 sPolynomial_eq_zero_of_left_eq_zero

For any polynomial \(g \in \operatorname{MvPolynomial}{\sigma }{R}\) and monomial order \(m\), the S-polynomial with zero as first argument vanishes:

\[ \operatorname{Sph}{0}{g} = 0 \]
Proof
Lemma 19 sPolynomial_eq_zero_of_right_eq_zero’

For any polynomial \(g \in \operatorname{MvPolynomial}{\sigma }{R}\) and monomial order \(m\), the S-polynomial with zero as second argument vanishes:

\[ \operatorname{Sph}{f}{0} = 0 \]
Proof
Theorem 20 div_set’
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Let \(G'' \subseteq R[\mathbf{X}]\) be a set of polynomials where every nonzero element has a unit leading coefficient:

\[ \forall g \in G'',\ \big(\mathrm{IsUnit}(\operatorname{LC}_m(g)) \lor g = 0\big) \]

Then for any polynomial \(p \in R[\mathbf{X}]\), there exists a remainder \(r\) satisfying:

\[ \mathsf{IsRemainder}_m\, p\, G''\, r \]

where \(\operatorname{LC}_m(g)\) denotes the leading coefficient of \(g\) under monomial order \(m\).

Proof
Theorem 21 div_set’’
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Let \(k\) be a field, and let \(G'' \subseteq k[x_i : i \in \sigma ]\) be a set of polynomials. Then for any \(p \in k[x_i : i \in \sigma ]\), there exists a generalized remainder \(r\) of \(p\) upon division by \(G''\).

Proof
Lemma 22 Ideal.fg_span_iff_fg_span_finset_subset
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A subset \(s \subseteq R\) has finitely generated span if and only if: \(\exists \) finite \(s' \subseteq s\) such that \(\mathsf{span}(s) = \mathsf{span}(s')\)

Proof
Lemma 23 span_singleton_zero
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For any ring \(R\), the span of the zero singleton set equals the zero submodule:

\[ \mathsf{span}_R \{ (0 : R)\} = \bot \]
Proof
Lemma 24 span_insert_zero
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For any subset \(s \subseteq R\) of a ring \(R\), inserting zero does not change the linear span:

\[ \mathsf{span}_R(\{ 0\} \cup s) = \mathsf{span}_R(s) \]
Proof
Lemma 25 span_sdiff_singleton_zero
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For any subset \(s \subseteq R\) of a ring \(R\), removing zero does not change the linear span:

\[ \mathsf{span}_R(s \setminus \{ 0\} ) = \mathsf{span}_R(s) \]
Proof
Lemma 26 leadingTerm_ideal_span_monomial

Let \(G'' \subseteq R[x_1, \dots , x_n]\) be a set of polynomials such that

\[ \forall p \in G'',\ \operatorname {leadingCoeff}(p) \in R^\times . \]

Then,

\[ \left\langle \operatorname {lt}(G'') \right\rangle = \left\langle x^{\deg (p)} \mid p \in G'' \right\rangle , \]
Proof
Lemma 27 leadingTerm_ideal_sdiff_singleton_zero
Proof
Lemma 28 leadingTerm_ideal_insert_zero
Proof
Lemma 29 IsGroebnerBasis_erase_zero
Proof
Lemma 30 IsGroebnerBasis_union_singleton_zero
Proof
Lemma 31 leadingTerm_ideal_span_monomial’
\[ \langle \mathrm{lt}(G) \rangle = \left\langle \left\{ x^t : t \in \{ \mathrm{multideg}(p) : p \in G \setminus \{ 0\} \} \right\} \right\rangle \]
Proof
Lemma 32 mem_ideal_of_remainder_mem_ideal

Let \(G'' \subseteq R[x_1, \dots , x_n]\), let \(I \subseteq R[x_1, \dots , x_n]\) be an ideal, and let \(p, r \in R[x_1, \dots , x_n]\). Suppose that:

  • \(G'' \subseteq I\),

  • \(r \in I\),

  • \(r\) is the remainder of \(p\) upon division by \(G''\).

Then,

\[ p \in I. \]
Proof
Lemma 33 remainder_mem_ideal_iff

Let \(R\) be a commutative ring, and let \(G'' \subseteq R[x_1, \dots , x_n]\), \(I \subseteq R[x_1, \dots , x_n]\) be an ideal, and \(p, r \in R[x_1, \dots , x_n]\). Assume that:

  • \(G'' \subseteq I\),

  • \(r\) is the remainder of \(p\) upon division by \(G''\).

Then,

\[ r \in I \quad \Longleftrightarrow \quad p \in I. \]
Proof
Lemma 34 remainder_sub_remainder_mem_ideal

Let \(I \subseteq k[x_i : i \in \sigma ]\) be an ideal, and let \(G \subseteq I\) be a finite subset. Suppose that \(r_1\) and \(r_2\) are generalized remainders of a polynomial \(p\) upon division by \(G\). Then,

\[ r_1 - r_2 \in I. \]
Proof
Lemma 35 IsRemainder_term_not_mem_leading_term_ideal
Proof
Lemma 36 IsRemainder_term_not_mem_leading_term_ideal’
Proof
Lemma 37 IsRemainder_monomial_not_mem_leading_term_ideal
Proof
Lemma 38 IsRemainder_monomial_not_mem_leading_term_ideal’
Proof
Theorem 39 exists_groebner_basis

Let \(I \subseteq k[x_1, \ldots , x_n]\) be an ideal. Then there exists a finite subset \(G = \{ g_1, \ldots , g_t\} \) of \(I\) such that \(G\) is a Gröbner basis for \(I\).

Proof
Theorem 40 groebner_basis_isRemainder_zero_iff_mem_span

Let \(G = \{ g_1, \dots , g_t\} \) be a Gröbner basis for an ideal \(I \subseteq k[x_1, \dots , x_n]\) and let \(f \in k[x_1, \dots , x_n]\). Then \(f \in I\) if and only if the remainder on division of \(f\) by \(G\) is zero.

Proof
Theorem 41 groebner_basis_isRemainder_zero_iff_mem_span’
Proof
Theorem 42 groebner_basis_zero_isRemainder_iff_mem_span
Proof
Lemma 43 groebner_basis_zero_isRemainder_iff_mem_span’
Proof
Lemma 44 remainder_zero
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Proof
Theorem 45 IsGroebnerBasis_iff

Let \(G = \{ g_1, \ldots , g_t\} \) be a finite subset of \(k[x_1, \ldots , x_n]\). Then \(G\) is a Gröbner basis for the ideal \(I = \langle G \rangle \) if and only if for every \(f \in I\), the remainder of \(f\) on division by \(G\) is zero.

Proof
Theorem 46 span_groebner_basis

Let \(G = \{ g_1, \ldots , g_t\} \) be a Gröbner basis for an ideal \(I \subseteq k[x_1, \ldots , x_n]\). Then \(G\) is a basis for the vector space \(I\) over \(k\).

Proof
Lemma 47 sPolynomial_decomposition
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Let \(f, h_1, \dots , h_m \in k[\mathbf{x}] \setminus \{ 0\} \), and suppose

\[ f = c_1 h_1 + \cdots + c_m h_m, \quad \text{with } c_i \in k. \]

If

\[ \mathrm{lm}(h_1) = \mathrm{lm}(h_2) = \cdots = \mathrm{lm}(h_i) {\gt} \mathrm{lm}(f), \]

then

\[ f = \sum _{1 \leq i {\lt} j \leq m} c_{i,j} S(h_i, h_j), \quad c_{i,j} \in k. \]

Furthermore, if \(S(h_i, h_j) \ne 0\), then \(\mathrm{lm}(h_i) {\gt} \mathrm{lm}(S(h_i, h_j))\).

Proof
Lemma 48 sPolynomial_degree_lt
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\(h_1, h_2 \in k[\mathbf{x}], lm(h_1) = lm(h_2), S(h_1, h_2) \ne 0\), then \(lm(S(h_1, h_2)) {\lt} lm(h_1)\).

Proof
Theorem 49 buchberger_criterion

A basis \(G = \{ g_1, \ldots , g_t \} \) for an ideal \(I\) is a Gröbner basis if and only if \(S(g_i, g_j) \to _G 0\) for all \(i \neq j\).

Proof