Links between an integral and its "improper" version #
In its current state, mathlib only knows how to talk about definite ("proper") integrals,
in the sense that it treats integrals over [x, +∞)
the same as it treats integrals over
[y, z]
. For example, the integral over [1, +∞)
is not defined to be the limit of
the integral over [1, x]
as x
tends to +∞
, which is known as an improper integral.
Indeed, the "proper" definition is stronger than the "improper" one. The usual counterexample
is x ↦ sin(x)/x
, which has an improper integral over [1, +∞)
but no definite integral.
Although definite integrals have better properties, they are hardly usable when it comes to computing integrals on unbounded sets, which is much easier using limits. Thus, in this file, we prove various ways of studying the proper integral by studying the improper one.
Definitions #
The main definition of this file is MeasureTheory.AECover
. It is a rather technical definition
whose sole purpose is generalizing and factoring proofs. Given an index type ι
, a countably
generated filter l
over ι
, and an ι
-indexed family φ
of subsets of a measurable space α
equipped with a measure μ
, one should think of a hypothesis hφ : MeasureTheory.AECover μ l φ
as
a sufficient condition for being able to interpret ∫ x, f x ∂μ
(if it exists) as the limit of ∫ x in φ i, f x ∂μ
as i
tends to l
.
When using this definition with a measure restricted to a set s
, which happens fairly often, one
should not try too hard to use a MeasureTheory.AECover
of subsets of s
, as it often makes proofs
more complicated than necessary. See for example the proof of
MeasureTheory.integrableOn_Iic_of_intervalIntegral_norm_tendsto
where we use (fun x ↦ oi x)
as a
MeasureTheory.AECover
w.r.t. μ.restrict (Iic b)
, instead of using (fun x ↦ Ioc x b)
.
Main statements #
MeasureTheory.AECover.lintegral_tendsto_of_countably_generated
: ifφ
is aMeasureTheory.AECover μ l
, wherel
is a countably generated filter, and iff
is a measurableENNReal
-valued function, then∫⁻ x in φ n, f x ∂μ
tends to∫⁻ x, f x ∂μ
asn
tends tol
MeasureTheory.AECover.integrable_of_integral_norm_tendsto
: ifφ
is aMeasureTheory.AECover μ l
, wherel
is a countably generated filter, iff
is measurable and integrable on eachφ n
, and if∫ x in φ n, ‖f x‖ ∂μ
tends to someI : ℝ
as n tends tol
, thenf
is integrableMeasureTheory.AECover.integral_tendsto_of_countably_generated
: ifφ
is aMeasureTheory.AECover μ l
, wherel
is a countably generated filter, and iff
is measurable and integrable (globally), then∫ x in φ n, f x ∂μ
tends to∫ x, f x ∂μ
asn
tends to+∞
.
We then specialize these lemmas to various use cases involving intervals, which are frequent in analysis. In particular,
MeasureTheory.integral_Ioi_of_hasDerivAt_of_tendsto
is a version of FTC-2 on the interval(a, +∞)
, giving the formula∫ x in (a, +∞), g' x = l - g a
ifg'
is integrable andg
tends tol
at+∞
.MeasureTheory.integral_Ioi_of_hasDerivAt_of_nonneg
gives the same result assuming thatg'
is nonnegative instead of integrable. Its automatic integrability in this context is proved inMeasureTheory.integrableOn_Ioi_deriv_of_nonneg
.MeasureTheory.integral_comp_smul_deriv_Ioi
is a version of the change of variables formula on semi-infinite intervals.MeasureTheory.tendsto_limUnder_of_hasDerivAt_of_integrableOn_Ioi
shows that a function whose derivative is integrable on(a, +∞)
has a limit at+∞
.MeasureTheory.tendsto_zero_of_hasDerivAt_of_integrableOn_Ioi
shows that an integrable function whose derivative is integrable on(a, +∞)
tends to0
at+∞
.
Versions of these results are also given on the intervals (-∞, a]
and (-∞, +∞)
, as well as
the corresponding versions of integration by parts.
A sequence φ
of subsets of α
is a MeasureTheory.AECover
w.r.t. a measure μ
and a filter
l
if almost every point (w.r.t. μ
) of α
eventually belongs to φ n
(w.r.t. l
), and if
each φ n
is measurable. This definition is a technical way to avoid duplicating a lot of
proofs. It should be thought of as a sufficient condition for being able to interpret
∫ x, f x ∂μ
(if it exists) as the limit of ∫ x in φ n, f x ∂μ
as n
tends to l
.
See for example MeasureTheory.AECover.lintegral_tendsto_of_countably_generated
,
MeasureTheory.AECover.integrable_of_integral_norm_tendsto
and
MeasureTheory.AECover.integral_tendsto_of_countably_generated
.
- ae_eventually_mem : ∀ᵐ (x : α) ∂μ, ∀ᶠ (i : ι) in l, x ∈ φ i
- measurableSet (i : ι) : MeasurableSet (φ i)
Instances For
Slight reformulation of
MeasureTheory.AECover.integral_tendsto_of_countably_generated
.
If f
is integrable on intervals Ioc (a i) (b i)
,
where a i
tends to -∞ and b i
tends to ∞, and
∫ x in a i .. b i, ‖f x‖ ∂μ
converges to I : ℝ
along a filter l
,
then f
is integrable on the interval (-∞, ∞)
If f
is integrable on intervals Ioc (a i) b
,
where a i
tends to -∞, and
∫ x in a i .. b, ‖f x‖ ∂μ
converges to I : ℝ
along a filter l
,
then f
is integrable on the interval (-∞, b)
If f
is integrable on intervals Ioc a (b i)
,
where b i
tends to ∞, and
∫ x in a .. b i, ‖f x‖ ∂μ
converges to I : ℝ
along a filter l
,
then f
is integrable on the interval (a, ∞)
Alias of MeasureTheory.integrableOn_Ioc_of_intervalIntegral_norm_bounded
.
Alias of MeasureTheory.integrableOn_Ioc_of_intervalIntegral_norm_bounded_left
.
Alias of MeasureTheory.integrableOn_Ioc_of_intervalIntegral_norm_bounded_right
.
If the derivative of a function defined on the real line is integrable close to +∞
, then
the function has a limit at +∞
.
If a function and its derivative are integrable on (a, +∞)
, then the function tends to zero
at +∞
.
Fundamental theorem of calculus-2, on semi-infinite intervals (a, +∞)
.
When a function has a limit at infinity m
, and its derivative is integrable, then the
integral of the derivative on (a, +∞)
is m - f a
. Version assuming differentiability
on (a, +∞)
and continuity at a⁺
.
Note that such a function always has a limit at infinity,
see tendsto_limUnder_of_hasDerivAt_of_integrableOn_Ioi
.
Fundamental theorem of calculus-2, on semi-infinite intervals (a, +∞)
.
When a function has a limit at infinity m
, and its derivative is integrable, then the
integral of the derivative on (a, +∞)
is m - f a
. Version assuming differentiability
on [a, +∞)
.
Note that such a function always has a limit at infinity,
see tendsto_limUnder_of_hasDerivAt_of_integrableOn_Ioi
.
A special case of integral_Ioi_of_hasDerivAt_of_tendsto
where we assume that f
is C^1 with
compact support.
When a function has a limit at infinity, and its derivative is nonnegative, then the derivative
is automatically integrable on (a, +∞)
. Version assuming differentiability
on (a, +∞)
and continuity at a⁺
.
When a function has a limit at infinity, and its derivative is nonnegative, then the derivative
is automatically integrable on (a, +∞)
. Version assuming differentiability
on [a, +∞)
.
When a function has a limit at infinity l
, and its derivative is nonnegative, then the
integral of the derivative on (a, +∞)
is l - g a
(and the derivative is integrable, see
integrable_on_Ioi_deriv_of_nonneg
). Version assuming differentiability on (a, +∞)
and
continuity at a⁺
.
When a function has a limit at infinity l
, and its derivative is nonnegative, then the
integral of the derivative on (a, +∞)
is l - g a
(and the derivative is integrable, see
integrable_on_Ioi_deriv_of_nonneg'
). Version assuming differentiability on [a, +∞)
.
When a function has a limit at infinity, and its derivative is nonpositive, then the derivative
is automatically integrable on (a, +∞)
. Version assuming differentiability
on (a, +∞)
and continuity at a⁺
.
When a function has a limit at infinity, and its derivative is nonpositive, then the derivative
is automatically integrable on (a, +∞)
. Version assuming differentiability
on [a, +∞)
.
When a function has a limit at infinity l
, and its derivative is nonpositive, then the
integral of the derivative on (a, +∞)
is l - g a
(and the derivative is integrable, see
integrable_on_Ioi_deriv_of_nonneg
). Version assuming differentiability on (a, +∞)
and
continuity at a⁺
.
When a function has a limit at infinity l
, and its derivative is nonpositive, then the
integral of the derivative on (a, +∞)
is l - g a
(and the derivative is integrable, see
integrable_on_Ioi_deriv_of_nonneg'
). Version assuming differentiability on [a, +∞)
.
If the derivative of a function defined on the real line is integrable close to -∞
, then
the function has a limit at -∞
.
If a function and its derivative are integrable on (-∞, a]
, then the function tends to zero
at -∞
.
Fundamental theorem of calculus-2, on semi-infinite intervals (-∞, a)
.
When a function has a limit m
at -∞
, and its derivative is integrable, then the
integral of the derivative on (-∞, a)
is f a - m
. Version assuming differentiability
on (-∞, a)
and continuity at a⁻
.
Note that such a function always has a limit at minus infinity,
see tendsto_limUnder_of_hasDerivAt_of_integrableOn_Iic
.
Fundamental theorem of calculus-2, on semi-infinite intervals (-∞, a)
.
When a function has a limit m
at -∞
, and its derivative is integrable, then the
integral of the derivative on (-∞, a)
is f a - m
. Version assuming differentiability
on (-∞, a]
.
Note that such a function always has a limit at minus infinity,
see tendsto_limUnder_of_hasDerivAt_of_integrableOn_Iic
.
A special case of integral_Iic_of_hasDerivAt_of_tendsto
where we assume that f
is C^1 with
compact support.
Fundamental theorem of calculus-2, on the whole real line
When a function has a limit m
at -∞
and n
at +∞
, and its derivative is integrable, then the
integral of the derivative is n - m
.
Note that such a function always has a limit at -∞
and +∞
,
see tendsto_limUnder_of_hasDerivAt_of_integrableOn_Iic
and
tendsto_limUnder_of_hasDerivAt_of_integrableOn_Ioi
.
If a function and its derivative are integrable on the real line, then the integral of the derivative is zero.
Change-of-variables formula for Ioi
integrals of vector-valued functions, proved by taking
limits from the result for finite intervals.
Change-of-variables formula for Ioi
integrals of scalar-valued functions
The substitution y = x ^ p
in integrals over Ioi 0
preserves integrability.
The substitution y = x ^ p
in integrals over Ioi 0
preserves integrability (version
without |p|
factor)
Integration by parts #
Integration by parts on (-∞, ∞).
With respect to a general bilinear form. For the specific case of multiplication, see
integral_mul_deriv_eq_deriv_mul
.
Integration by parts on (-∞, ∞). With respect to a general bilinear form, assuming moreover that the total function is integrable.
For finite intervals, see: intervalIntegral.integral_deriv_mul_eq_sub
.
Integration by parts on (-∞, ∞).
For finite intervals, see: intervalIntegral.integral_mul_deriv_eq_deriv_mul
.
Integration by parts on (-∞, ∞). Version assuming that the total function is integrable
For finite intervals, see: intervalIntegral.integral_deriv_mul_eq_sub
.
Integration by parts on (a, ∞).
For finite intervals, see: intervalIntegral.integral_mul_deriv_eq_deriv_mul
.
For finite intervals, see: intervalIntegral.integral_deriv_mul_eq_sub
.
Integration by parts on (∞, a].
For finite intervals, see: intervalIntegral.integral_mul_deriv_eq_deriv_mul
.