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 in the archive).

If you are managing your Coq installation using OPAM, you can install the library using the following command:

opam install --jobs=2 coq-interval

Note that the `coq-interval`

package is hosted in the OPAM
repository dedicated
to Coq libraries.
So you have to type the following command beforehand, if your OPAM
installation does not yet know about this repository.

opam repo add coq-released https://coq.inria.fr/opam/released

- version 4.0.0 (Coq 8.8 to 8.12, Flocq 3.1, MathComp 1.6, Coquelicot 3.0),
- version 3.4.2 (Coq 8.7 to 8.11, Flocq 3.0, MathComp 1.6, Coquelicot 3.0),
- version 3.3.0 (Coq 8.5 to 8.8, Flocq 2.5, MathComp 1.6, Coquelicot 3.0),
- version 3.2.0 (Coq 8.5 & 8.6, Flocq 2.5, MathComp 1.6, Coquelicot 3.0),
- version 3.1.1 (Coq 8.5 & 8.6, Flocq 2.5, MathComp 1.6, Coquelicot),
- version 2.2.1 (Coq 8.4 & 8.5, Flocq 2.5, MathComp 1.6, Coquelicot).

In addition to the Coq proof assistant, you need the following libraries:

- Mathematical Components,
- Flocq,
- Coquelicot,
- BigNums (for Coq >= 8.7).

This library is hosted on the Inria Gitlab server. It was mainly developed by Guillaume Melquiond.

Ideally, you should just have to type:

./configure && ./remake --jobs=2 && ./remake install

The environment variable `COQC`

can be passed to the
configure script in order to set the Coq compiler command. The configure
script defaults to `coqc`

. Similarly, `COQDEP`

can
be used to specify the location of `coqdep`

. The
`COQBIN`

environment variable can be used to set both
variables at once.

Option `--libdir=DIR`

sets the directory where the compiled
library files should be installed by `./remake install`

. By
default, the target directory is ```
`$COQC
-where`/user-contrib/Interval
```

.

The files are compiled at a logical location starting with
`Interval`

.

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 <= c1`

,`c1 <= e`

,`e >= c1`

, and`c1 >= e`

;`e < c1`

,`c1 < e`

,`e > c1`

, and`c1 > e`

;`e <> c1`

and`c1 <> e`

;`Rabs e <= c1`

, handled as`-c1 <= e <= c1`

.

The complete list of recognized hypotheses is as follows:

`c1 <= t <= c2`

;`t <= c1`

,`c1 <= t`

,`t >= c1`

, and`c1 >= t`

;`Rabs t <= c1`

.

The tactics recognize the following operators: `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:

`powerRZ t _ * (ln t) ^ _`

,`/ (t * (ln t) ^ _)`

.

The helper tactic `integral_intro`

is the counterpart
of `interval_intro`

, but for introducing enclosures of
integrals into the proof context. As with `interval_intro`

,
keywords `lower`

, `upper`

, and `as`

, are
supported.

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 used to emulate floating-point computations. If
this parameter is not specified, the tactics perform computations
using machine floating-point numbers, when available. Otherwise, the
tactic defaults to using
`i_prec 53`

. Note that, in some corner cases, the tactics might fail when using native numbers, despite the goals being provable using a 53-bit emulation. `i_native_compute`

- 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`

.

(** BEGIN **) 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. apply Rminus_lt. 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 with (i_prec 10). Qed. (*** END ***)