Micromega: tactics for solving arithmetic goals over ordered rings — Coq 8.8.1 documentation

# Micromega: tactics for solving arithmetic goals over ordered rings

Authors: | Frédéric Besson and Evgeny Makarov

## Short description of the tactics

The Psatz module (`Require Import Psatz.`) gives access to several tactics for solving arithmetic goals over \(\mathbb{Z}\), \(\mathbb{Q}\), and \(\mathbb{R}\) [1]. It also possible to get the tactics for integers by a `Require Import Lia`, rationals `Require Import Lqa` and reals `Require Import Lra`.

- `lia` is a decision procedure for linear integer arithmetic;
- `nia` is an incomplete proof procedure for integer non-linear arithmetic;
- `lra` is a decision procedure for linear (real or rational) arithmetic;
- `nra` is an incomplete proof procedure for non-linear (real or rational) arithmetic;
- `psatz` `D n` where `D` is \(\mathbb{Z}\) or \(\mathbb{Q}\) or \(\mathbb{R}\), and `n` is an optional integer limiting the proof search depth is an incomplete proof procedure for non-linear arithmetic. It is based on John Harrison’s HOL Light driver to the external prover csdp [2]. Note that the csdp driver is generating a proof cache which makes it possible to rerun scripts even without csdp.

The tactics solve propositional formulas parameterized by atomic arithmetic expressions interpreted over a domain \(D\) ∈ {ℤ, ℚ, ℝ}. The syntax of the formulas is the following:

>     F ::=  A ∣ P ∣ True ∣ False ∣ F 1 ∧ F 2 ∣ F 1 ∨ F 2 ∣ F 1 ↔ F 2 ∣ F 1 → F 2 ∣ ¬ F
>     A ::=  p 1 = p 2 ∣ p 1 > p 2 ∣ p 1 < p 2 ∣ p 1 ≥ p 2 ∣ p 1 ≤ p 2
>     p ::=  c ∣ x ∣ −p ∣ p 1 − p 2 ∣ p 1 + p 2 ∣ p 1 × p 2 ∣ p ^ n

where \(c\) is a numeric constant, \(x \in D\) is a numeric variable, the operators \(−, +, ×\) are respectively subtraction, addition, and product; \(p ^ n\) is exponentiation by a constant \(n\), \(P\) is an arbitrary proposition. For \(\mathbb{Q}\), equality is not Leibniz equality = but the equality of rationals ==.

For \(\mathbb{Z}\) (resp. \(\mathbb{Q}\)), \(c\) ranges over integer constants (resp. rational constants). For \(\mathbb{R}\), the tactic recognizes as real constants the following expressions:

    c ::= R0 | R1 | Rmul(c,c) | Rplus(c,c) | Rminus(c,c) | IZR z | IQR q | Rdiv(c,c) | Rinv c

where \(z\) is a constant in \(\mathbb{Z}\) and \(q\) is a constant in \(\mathbb{Q}\). This includes integer constants written using the decimal notation, i.e., c%R.

## Positivstellensatz refutations

The name psatz is an abbreviation for positivstellensatz – literally "positivity theorem" – which generalizes Hilbert’s nullstellensatz. It relies on the notion of Cone. Given a (finite) set of polynomials \(S\), \(\mathit{Cone}(S)\) is inductively defined as the smallest set of polynomials closed under the following rules:

\(\begin{array}{l} \dfrac{p \in S}{p \in \mathit{Cone}(S)} \quad \dfrac{}{p^2 \in \mathit{Cone}(S)} \quad \dfrac{p_1 \in \mathit{Cone}(S) \quad p_2 \in \mathit{Cone}(S) \quad \Join \in \{+,*\}} {p_1 \Join p_2 \in \mathit{Cone}(S)}\\ \end{array}\)

The following theorem provides a proof principle for checking that a set of polynomial inequalities does not have solutions [3].

Theorem (Psatz). Let \(S\) be a set of polynomials. If \(-1\) belongs to \(\mathit{Cone}(S)\), then the conjunction \(\bigwedge_{p \in S} p\ge 0\) is unsatisfiable. A proof based on this theorem is called a positivstellensatz refutation. The tactics work as follows. Formulas are normalized into conjunctive normal form \(\bigwedge_i C_i\) where \(C_i\) has the general form \((\bigwedge_{j\in S_i} p_j \Join 0) \to \mathit{False})\) and \(\Join \in \{>,\ge,=\}\) for \(D\in \{\mathbb{Q},\mathbb{R}\}\) and \(\Join \in \{\ge, =\}\) for \(\mathbb{Z}\).

For each conjunct \(C_i\), the tactic calls a oracle which searches for \(-1\) within the cone. Upon success, the oracle returns a cone expression that is normalized by the ring tactic (see The ring and field tactic families) and checked to be \(-1\).

## lra: a decision procedure for linear real and rational arithmetic

`lra`

This tactic is searching for linear refutations using Fourier elimination [4]. As a result, this tactic explores a subset of the Cone defined as

> \(\mathit{LinCone}(S) =\left\{ \left. \sum_{p \in S} \alpha_p \times p~\right|~\alpha_p \mbox{ are positive constants} \right\}\)

The deductive power of lra is the combined deductive power of ring_simplify and fourier. There is also an overlap with the field tactic e.g., \(x = 10 * x / 10\) is solved by lra.

## lia: a tactic for linear integer arithmetic

`lia`

This tactic offers an alternative to the `omega` and `romega` tactics. Roughly speaking, the deductive power of lia is the combined deductive power of `ring_simplify` and `omega`. However, it solves linear goals that `omega` and `romega` do not solve, such as the following so-called omega nightmare [Pug92].

Goal forall x y, 27 <= 11 * x + 13 * y <= 45 -> -10 <= 7 * x - 9 * y <= 4 -> False.
  Toplevel input, characters 52-55: > Goal forall x y, 27 <= 11 * x + 13 * y <= 45 -> -10 <= 7 * x - 9 * y <= 4 -> False. > ^^^ Error: Cannot interpret a negative number as a number of type nat

The estimation of the relative efficiency of `lia` vs `omega` and `romega` is under evaluation.

### High level view of lia

Over \(\mathbb{R}\), positivstellensatz refutations are a complete proof principle [5]. However, this is not the case over \(\mathbb{Z}\). Actually, positivstellensatz refutations are not even sufficient to decide linear integer arithmetic. The canonical example is \(2 * x = 1 -> \mathtt{False}\) which is a theorem of \(\mathbb{Z}\) but not a theorem of \({\mathbb{R}}\). To remedy this weakness, the lia tactic is using recursively a combination of:

- linear positivstellensatz refutations;
- cutting plane proofs;
- case split.

### Cutting plane proofs

are a way to take into account the discreteness of \(\mathbb{Z}\) by rounding up (rational) constants up-to the closest integer.

Theorem `Bound on the ceiling function`

  Let \(p\) be an integer and \(c\) a rational constant. Then \(p \ge c \rightarrow p \ge \lceil{c}\rceil\).

For instance, from 2 x = 1 we can deduce

- \(x \ge 1/2\) whose cut plane is \(x \ge \lceil{1/2}\rceil = 1\);
- \(x \le 1/2\) whose cut plane is \(x \le \lfloor{1/2}\rfloor = 0\).

By combining these two facts (in normal form) \(x − 1 \ge 0\) and \(-x \ge 0\), we conclude by exhibiting a positivstellensatz refutation: \(−1 \equiv x−1 + −x \in \mathit{Cone}({x−1,x})\).

Cutting plane proofs and linear positivstellensatz refutations are a complete proof principle for integer linear arithmetic.

### Case split

enumerates over the possible values of an expression.

Theorem. Let \(p\) be an integer and \(c_1\) and \(c_2\) integer constants. Then:

> \(c_1 \le p \le c_2 \Rightarrow \bigvee_{x \in [c_1,c_2]} p = x\)

Our current oracle tries to find an expression \(e\) with a small range \([c_1,c_2]\). We generate \(c_2 − c_1\) subgoals which contexts are enriched with an equation \(e = i\) for \(i \in [c_1,c_2]\) and recursively search for a proof.

## nra: a proof procedure for non-linear arithmetic

`nra`

This tactic is an experimental proof procedure for non-linear arithmetic. The tactic performs a limited amount of non-linear reasoning before running the linear prover of lra. This pre-processing does the following:

- If the context contains an arithmetic expression of the form \(e[x^2]\) where \(x\) is a monomial, the context is enriched with \(x^2 \ge 0\);
- For all pairs of hypotheses \(e_1 \ge 0\), \(e_2 \ge 0\), the context is enriched with \(e_1 \times e_2 \ge 0\).

After this pre-processing, the linear prover of lra searches for a proof by abstracting monomials by variables.

## nia: a proof procedure for non-linear integer arithmetic

`nia`

This tactic is a proof procedure for non-linear integer arithmetic. It performs a pre-processing similar to nra. The obtained goal is solved using the linear integer prover lia.

## psatz: a proof procedure for non-linear arithmetic

`psatz`

This tactic explores the \(\mathit{Cone}\) by increasing degrees – hence the depth parameter \(n\). In theory, such a proof search is complete – if the goal is provable the search eventually stops. Unfortunately, the external oracle is using numeric (approximate) optimization techniques that might miss a refutation.

To illustrate the working of the tactic, consider we wish to prove the following Coq goal:

Require Import ZArith Psatz.
  [Loading ML file z_syntax_plugin.cmxs ... done] [Loading ML file quote_plugin.cmxs ... done] [Loading ML file newring_plugin.cmxs ... done] [Loading ML file omega_plugin.cmxs ... done] [Loading ML file r_syntax_plugin.cmxs ... done] [Loading ML file micromega_plugin.cmxs ... done]
Open Scope Z_scope.
Goal forall x, -x^2 >= 0 -> x - 1 >= 0 -> False.
  1 subgoal ============================ forall x : Z, - x ^ 2 >= 0 -> x - 1 >= 0 -> False
intro x.
  1 subgoal x : Z ============================ - x ^ 2 >= 0 -> x - 1 >= 0 -> False
psatz Z 2.
  Toplevel input, characters 0-9: > psatz Z 2. > ^^^^^^^^^ Error: In nested Ltac calls to "psatz (constr) (int_or_var)", "xpsatz" and "psatz_Z (int_or_var) (tactic)", last call failed. Tactic failure: Skipping what remains of this tactic: the complexity of the goal requires the use of a specialized external tool called csdp. Unfortunately Coq isn't aware of the presence of any "csdp" executable in the path. Csdp packages are provided by some OS distributions; binaries and source code can be downloaded from https://projects.coin-or.org/Csdp.

As shown, such a goal is solved by `intro x. psatz Z 2.`. The oracle returns the cone expression \(2 \times (x-1) + (\mathbf{x-1}) \times (\mathbf{x−1}) + -x^2\) (polynomial hypotheses are printed in bold). By construction, this expression belongs to \(\mathit{Cone}({−x^2,x -1})\). Moreover, by running ring we obtain \(-1\). By Theorem Psatz, the goal is valid.

[1] | Support for nat and \(\mathbb{N}\) is obtained by pre-processing the goal with the zify tactic.

[2] | Sources and binaries can be found at https://projects.coin-or.org/Csdp

[3] | Variants deal with equalities and strict inequalities.

[4] | More efficient linear programming techniques could equally be employed.

[5] | In practice, the oracle might fail to produce such a refutation.