JuliaApproximation / JuliaApproximation/ApproxFun.jl

Discrepancy in number of points and number of values on tensor spaces

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Description

When trying to use ApproxFun to interpolate a function on Chebyshev-points and compute derivatives, I came across the following behaviour:

Define two spaces of Chebyshev and ultraspherical polynomials:

using ApproxFun
C2 = Chebyshev(0..1)^2
U2 = Ultraspherical(1, 0..1)^2

Project a function onto the Chebyshev space:

> fc = Fun((x,y) -> cos(π*x) * cos(π*y), C2)
> (length(points(fc)), length(values(fc)), ncoefficients(fc))
(190, 190, 185)

Transform to ultraspherical space:

> fu = Conversion(C2, U2) * fc
> (length(points(fu)), length(values(fu)), ncoefficients(fu))
(196, 361, 190)

According to the docs, values(f::Fun) returns the f::Fun evaluated on points(f), but that does not seem to be the case as the number of points and values are very different. I am wondering what exactly is happening here.

The result is similar when projecting onto the ultraspherical space right away

> fu2 = Fun((x,y) -> cos(π*x) * cos(π*y), U2)
> (length(points(fu2)), length(values(fu2)), ncoefficients(fu2))
(196, 361, 185)

I am also not quite sure why ncoefficients(f) is smaller than length(points(f)).

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Research direction

Start by reproducing the tensor-space examples with Fun, Chebyshev, Ultraspherical, Conversion, points, values, and ncoefficients. Inspect how these entry points represent points, values, and coefficients for two-dimensional spaces, then establish whether the behavior or the documentation is incorrect. Done means the discrepancy and the relationship among the reported lengths are clearly explained and verified.

Written by the indexing model from the issue text.

Assessment

Tech stack
julia
Domain
tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
30/100

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