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For the official repo URL, see https://gitlab.inria.fr/coqinterval/interval.

CoqInterval

This library provides vernacular files containing tactics for simplifying the proofs of inequalities on expressions of real numbers for the Coq proof assistant.

This package is free software; you can redistribute it and/or modify it under the terms of CeCILL-C Free Software License (see the COPYING file). Main author is Guillaume Melquiond guillaume.melquiond@inria.fr.

See the file INSTALL.md for installation instructions.

Project Home

Homepage: http://coq-interval.gforge.inria.fr/

Repository: https://gitlab.inria.fr/coqinterval/interval

Bug tracker: https://gitlab.inria.fr/coqinterval/interval/issues

Invocation

In order to use the tactics of the library, one has to import the Interval.Tactic file into a Coq proof script. The main tactic is named interval.

The tactic can be applied on a goal of the form c1 <= e <= c2 with e an expression involving real-valued operators. Sub-expressions that are not recognized by the tactic should be either terms t appearing in hypothesis inequalities c3 <= t <= c4 or simple integers. The bounds c1, c2, etc, are expressions that contain only constant leaves, e.g., 5 / sqrt (1 + PI).

The complete list of recognized goals is as follows:

• c1 <= e <= c2;
• e <= c2;
• c1 <= e;
• 0 < e;
• e <> 0;
• Rabs e <= c2, handled as -c2 <= e <= c2;
• e1 <= e2, handled as e1 - e2 <= 0;
• e1 < e2, handled as 0 < e2 - e1;
• e1 <> e2, handled as e1 - e2 <> 0.

Operators recognized by the tactic are PI, Ropp, Rabs, Rinv, Rsqr, sqrt, cos, sin, tan, atan, exp, ln, pow, powerRZ, Rplus, Rminus, Rmult, Rdiv. Operators Zfloor, Zceil, Ztrunc, ZnearestE (composed with IZR) are also recognized. There are some restrictions on the domain of a few functions: pow and powerRZ should be written with a numeric exponent; the input of cos and sin should be between -2*PI and 2*PI; the input of tan should be between -PI/2 and PI/2. Outside of these domains, the trigonometric functions return meaningful results only for singleton input intervals.

A helper tactic interval_intro e is also available. Instead of proving the current goal, it computes an enclosure of the expression e passed as argument and it introduces the resulting inequalities into the proof context. If only one bound is needed, the keywords lower and upper can be passed to the tactic, so that it does not perform useless computations. For example, interval_intro e lower introduces only the inequality corresponding to the lower bound of e in the context. The interval_intro tactic uses a fresh name for the generated inequalities, unless one uses as followed by an intro pattern.

The integral tactic is dedicated to verifying enclosures of integrals. Such an integral should be expressed using RInt; its bounds should be constant; and its integrand should be an expression containing only constant leaves except for the integration variable. Improper integrals are also supported, when expressed using RInt_gen. The supported bounds are then (at_right 0) (at_point _) and (at_point _) (Rbar_locally p_infty). In the improper case, the integrand should be of the form (fun t => f t * g t) with f a function bounded on the integration domain and g one of the following expressions:

• exp (- (_ * t)),
• powerRZ t _ * (ln t) ^ _,
• / (t * (ln t) ^ _).

The helper tactic integral_intro is the pendant of interval_intro, but for introducing enclosures of integrals into the proof context. As with interval_intro, keywords lower, upper, and as, are supported.

Fine-tuning

The behavior of the tactics can be tuned by passing an optional set of parameters with (param1, param2, ...). These parameters are parsed from left to right. If some parameters are conflicting, the earlier ones are discarded. Available parameters are as follows (with the type of their arguments, if any):

• i_prec (p:positive)

  Set the precision of the floating-point computations. Default precision is 30 bits.

• i_native_compute

  Instruct the tactics to perform computations using native_compute instead of vm_compute. This greatly increases the startup time of the tactics, but makes the computations faster. This is useful only for computationally-intensive proofs.

• i_bisect (x:R)

  Instruct the tactics to split the interval enclosing x until the goal is proved on all the sub-intervals. Several i_bisect parameters can be given. In that case, the tactic cycles through all of them, splitting the input domain along the corresponding variable. Computation time is more or less proportional to the final number of sub-domains. This parameter is only meaningful for the interval and interval_intro tactics.

• i_depth (n:nat)

  Set the maximal bisection depth. Setting it to a nonzero value has no effect unless i_bisect parameters are also passed. If the maximal depth is n, the tactic will consider up to 2^n sub-domains in the worst case. As with i_bisect, this parameter is only meaningful for the interval and interval_intro tactics. The maximal depth defaults to 15 for interval, and to 5 for interval_intro. Note that interval_intro computes the best enclosure that could be verified by interval using the same maximal depth.

• i_autodiff (x:R)

  Instruct the tactics to perform an automatic differentiation of the target expression with respect to x. This makes the tactic about twice slower on each sub-domain. But it makes it possible to detect some monotony properties of the target expression, thus reducing the amount of sub-domains that need to be considered. Note that this is only useful if there are several occurrences of x in the goal. This parameter is only meaningful for the interval and interval_intro tactics. It is mutually exclusive with i_taylor.

• i_taylor (x:R)

  Instruct the tactics to compute a reliable polynomial enclosure of the target expression using Taylor models in x. As with i_autodiff, this is useful only if x occurs several times in the goal. Computing polynomial enclosures is much slower than automatic differentiation, but it can reduce the final number of sub-domains even further, thus speeding up proofs. Note that it might fail to prove goals that are feasible using automatic differentiation. As with i_autodiff, the i_taylor parameter is only meaningful for the interval and interval_intro tactics. It is implicit for the integral and integral_intro tactics, as Taylor models of the integrand are computed with respect to the integration variable.

• i_degree (d:nat)

  Set the degree of polynomials used as enclosures. The default degree is 10. For interval and interval_intro, this parameter is only meaningful in conjunction with i_taylor.

• i_fuel (n:positive)

  Set the maximum number of sub-domains considered when bounding integrals. The tactics maintain a set of integration sub-domains; it splits the sub-domains that contribute the most to the inaccuracy of the integral until its enclosure is tight enough to satisfy the goal. By default, the tactics will split the integration domain into at most 100 sub-domains. This parameter is only meaningful for the integral and integral_intro tactics.

• i_width (p:Z)

  Instruct the integral_intro tactic to compute an enclosure of the integral that is no larger than 2^p. The tactic will split the integration domain until the resulting enclosure reaches this width or i_fuel is exhausted. This parameter is meaningless for the other tactics. It is mutually exclusive with i_relwidth.

• i_relwidth (p:positive)

  Instruct the integral_intro tactic to compute an enclosure of the integral whose relative width is no larger than 2^-p. This parameter is meaningless for the other tactics. It defaults to 10. This means that, if neither i_width nor i_relwidth is used, integral_intro will compute an enclosure of the integral accurate to three decimal digits, assuming i_fuel is large enough.

• i_delay

  Prevent Coq from verifying the generated proof at invocation time. Instead, Coq will check the proof term at Qed time. This makes the tactics interval and integral instant. But it also means that failures, if any, will only be detected at Qed time, possibly with an inscrutable error message. This parameter is thus meant to be used when editing a proof script for which the tactics are already known to succeed. For the tactics interval_intro and integral_intro, computations are still performed (the risk of failure is thus negligible), but the i_delay parameter delays their verification to Qed time. This makes these tactics twice as fast. This is especially useful when looking for optimal values for parameters such as i_prec and i_degree.

Examples

Require Import Reals.
Require Import Interval.Tactic.

Open Scope R_scope.

Goal
  forall x, -1 <= x <= 1 ->
  sqrt (1 - x) <= 3/2.
Proof.
  intros.
  interval.
Qed.

Goal
  forall x, -1 <= x <= 1 ->
  sqrt (1 - x) <= 141422/100000.
Proof.
  intros.
  interval.
Qed.

Goal
  forall x, -1 <= x <= 1 ->
  sqrt (1 - x) <= 141422/100000.
Proof.
  intros.
  interval_intro (sqrt (1 - x)) upper as H'.
  apply Rle_trans with (1 := H').
  interval.
Qed.

Goal
  forall x, 3/2 <= x <= 2 ->
  forall y, 1 <= y <= 33/32 ->
  Rabs (sqrt(1 + x/sqrt(x+y)) - 144/1000*x - 118/100) <= 71/32768.
Proof.
  intros.
  interval with (i_prec 19, i_bisect x).
Qed.

Goal
  forall x, 1/2 <= x <= 2 ->
  Rabs (sqrt x - (((((122 / 7397 * x + (-1733) / 13547) * x
                   + 529 / 1274) * x + (-767) / 999) * x
                   + 407 / 334) * x + 227 / 925))
    <= 5/65536.
Proof.
  intros.
  interval with (i_bisect x, i_taylor x, i_degree 3).
Qed.

Goal
  forall x, -1 <= x ->
  x < 1 + powerRZ x 3.
Proof.
  intros.
  interval with (i_bisect x, i_autodiff x).
Qed.

Require Import Coquelicot.Coquelicot.

Goal
  Rabs (RInt (fun x => atan (sqrt (x*x + 2)) / (sqrt (x*x + 2) * (x*x + 1))) 0 1
        - 5/96*PI*PI) <= 1/1000.
Proof.
  integral with (i_fuel 2, i_degree 5).
Qed.

Goal
  RInt_gen (fun x => 1 * (powerRZ x 3 * ln x^2))
           (at_right 0) (at_point 1) = 1/32.
Proof.
  refine ((fun H => Rle_antisym _ _ (proj2 H) (proj1 H)) _).
  integral.
Qed.