We specialize the theory fo analytic functions to the case of functions that admit a
development given by a finite formal multilinear series. We call them "continuously polynomial",
which is abbreviated to CPolynomial
. One reason to do that is that we no longer need a
completeness assumption on the target space F
to make the series converge, so some of the results
are more general. The class of continuously polynomial functions includes functions defined by
polynomials on a normed ๐
-algebra and continuous multilinear maps.
Main definitions #
Let p
be a formal multilinear series from E
to F
, i.e., p n
is a multilinear map on E^n
for n : โ
, and let f
be a function from E
to F
.
HasFiniteFPowerSeriesOnBall f p x n r
: on the ball of centerx
with radiusr
,f (x + y) = โ'_n pโ yแต
, and moreoverpโ = 0
ifn โค m
.HasFiniteFPowerSeriesAt f p x n
: on some ball of centerx
with positive radius, holdsHasFiniteFPowerSeriesOnBall f p x n r
.CPolynomialAt ๐ f x
: there exists a power seriesp
and a natural numbern
such that holdsHasFPowerSeriesAt f p x n
.CPolynomialOn ๐ f s
: the functionf
is analytic at every point ofs
.
We develop the basic properties of these notions, notably:
- If a function is continuously polynomial, then it is analytic, see
HasFiniteFPowerSeriesOnBall.hasFPowerSeriesOnBall
,HasFiniteFPowerSeriesAt.hasFPowerSeriesAt
,CPolynomialAt.analyticAt
andCPolynomialOn.analyticOnNhd
. - The sum of a finite formal power series with positive radius is well defined on the whole space,
see
FormalMultilinearSeries.hasFiniteFPowerSeriesOnBall_of_finite
. - If a function admits a finite power series in a ball, then it is continuously polynomial at
any point
y
of this ball, and the power series there can be expressed in terms of the initial power seriesp
asp.changeOrigin y
, which is finite (with the same bound asp
) bychangeOrigin_finite_of_finite
. SeeHasFiniteFPowerSeriesOnBall.changeOrigin
. It follows in particular that the set of points at which a given function is continuously polynomial is open, seeisOpen_cPolynomialAt
.
We prove in particular that continuous multilinear maps are continuously polynomial, and so are continuous linear maps into continuous multilinear maps. In particular, such maps are analytic.
Given a function f : E โ F
, a formal multilinear series p
and n : โ
, we say that
f
has p
as a finite power series on the ball of radius r > 0
around x
if
f (x + y) = โ' pโ yแต
for all โyโ < r
and pโ = 0
for n โค m
.
Instances For
Given a function f : E โ F
, a formal multilinear series p
and n : โ
, we say that
f
has p
as a finite power series around x
if f (x + y) = โ' pโ yโฟ
for all y
in a
neighborhood of 0
and pโ = 0
for n โค m
.
Equations
- HasFiniteFPowerSeriesAt f p x n = โ (r : ENNReal), HasFiniteFPowerSeriesOnBall f p x n r
Instances For
Given a function f : E โ F
, we say that f
is continuously polynomial (cpolynomial)
at x
if it admits a finite power series expansion around x
.
Equations
- CPolynomialAt ๐ f x = โ (p : FormalMultilinearSeries ๐ E F) (n : โ), HasFiniteFPowerSeriesAt f p x n
Instances For
Given a function f : E โ F
, we say that f
is continuously polynomial on a set s
if it is continuously polynomial around every point of s
.
Equations
- CPolynomialOn ๐ f s = โ x โ s, CPolynomialAt ๐ f x
Instances For
If a function f
has a finite power series p
around x
, then the function
z โฆ f (z - y)
has the same finite power series around x + y
.
If a function f
has a finite power series p
on a ball and g
is a continuous linear map,
then g โ f
has the finite power series g โ p
on the same ball.
If a function f
is continuously polynomial on a set s
and g
is a continuous linear map,
then g โ f
is continuously polynomial on s
.
If a function admits a finite power series expansion bounded by n
, then it is equal to
the m
th partial sums of this power series at every point of the disk for n โค m
.
Variant of the previous result with the variable expressed as y
instead of x + y
.
The particular cases where f
has a finite power series bounded by 0
or 1
.
If f
has a formal power series on a ball bounded by 0
, then f
is equal to 0
on
the ball.
If f
has a formal power series at x
bounded by 0
, then f
is equal to 0
in a
neighborhood of x
.
If f
has a formal power series on a ball bounded by 1
, then f
is constant equal
to f x
on the ball.
If f
has a formal power series at x bounded by 1
, then f
is constant equal
to f x
in a neighborhood of x
.
If a function admits a finite power series expansion on a disk, then it is continuous there.
Continuously polynomial everywhere implies continuous
A finite formal multilinear series sums to its sum at every point.
The sum of a finite power series p
admits p
as a power series.
The sum of a finite power series is continuous.
We study what happens when we change the origin of a finite formal multilinear series p
. The
main point is that the new series p.changeOrigin x
is still finite, with the same bound.
If p
is a formal multilinear series such that p m = 0
for n โค m
, then
p.changeOriginSeriesTerm k l = 0
for n โค k + l
.
If p
is a finite formal multilinear series, then so is p.changeOriginSeries k
for every
k
in โ
. More precisely, if p m = 0
for n โค m
, then p.changeOriginSeries k m = 0
for
n โค k + m
.
If p
is a formal multilinear series such that p m = 0
for n โค m
, then
p.changeOrigin x k = 0
for n โค k
.
The terms of the formal multilinear series p.changeOrigin
are continuously polynomial
as we vary the origin
If a function admits a finite power series expansion p
on an open ball B (x, r)
, then
it is continuously polynomial at every point of this ball.
For any function f
from a normed vector space to a normed vector space, the set of points
x
such that f
is continuously polynomial at x
is open.
If f
is continuously polynomial at a point, then it is continuously polynomial in a
nonempty ball around that point.
Continuous multilinear maps #
We show that continuous multilinear maps are continuously polynomial, and therefore analytic.
Alias of ContinuousMultilinearMap.analyticOn
.
Continuous linear maps into continuous multilinear maps #
We show that a continuous linear map into continuous multilinear maps is continuously polynomial (as a function of two variables, i.e., uncurried). Therefore, it is also analytic.
Formal multilinear series associated to a linear map into multilinear maps.
Equations
- f.toFormalMultilinearSeriesOfMultilinear n = if h : Fintype.card (Option ฮน) = n then ContinuousMultilinearMap.domDomCongr (Fintype.equivFinOfCardEq h) f.continuousMultilinearMapOption else 0
Instances For
Alias of ContinuousLinearMap.analyticOn_uncurry_of_multilinear
.