Non-uniform grids

By employing non-uniform grids, more effort can be concentrated to those parts where the function is expected to vary more, hopefully leading to a better approximation. StaggeredFiniteDifferences supports both uniform and non-uniform node distributions; in the former case, the node locations are stored as an AbstractRange, in the latter, as any other kind of AbstractVector.


Since the quadrature weights in the non-uniform case are location-dependent, they are baked into to the expansion coefficients (as in the FE-DVR case), such that

\[\int\diff{x} f(x) \approx \sum_i f_i,\]

where $f_i$ is the $i$th expansion coefficient of $f(x)$. In the uniform case, the expansion coefficients instead equal the function value at the nodes, which means

\[\int\diff{x} f(x) \approx \Delta x \sum_i f_i.\]

To be sure that you always get the right integration weight, use the metric to perform integrals, e.g.

\[\int\diff{x} f(x)\]

is computed using

integral = sum(S*f)

where the metric matrix can be found using

S = B'B

Similarly, integrals of the kind

\[\int\diff{x} \conj{f}(x)g(x)\]

are easily computed as

integral = dot(f, S, g)


We start by constructing two StaggeredFiniteDifferences grids, one uniform and one log–linear, of approximately the same extent (at least including $r=30$):

julia> N = 50

julia> rmax = 30.0

julia> B1 = StaggeredFiniteDifferences(rmax, N)
Staggered finite differences basis {Float64} on 0.0..30.606060606060606 with 50 points spaced by ρ = 0.6060606060606061

julia> ρmin = 0.001 # Min step-size

julia> ρmax = 1.0 # Max step-size

julia> α = 0.1 # Step-size change rate

julia> B2 = StaggeredFiniteDifferences(ρmin, ρmax, α, rmax)
Staggered finite differences basis {Float64} on 0.0..31.475628188861677 with 103 points

As mentioned above, the metrics are different in the uniform and non-uniform cases:

julia> S1 = B1'B1

julia> S2 = B2'B2

Since the step-size is changing for non-uniform grids, step is ill-defined, but returns unity to simplify the implementation of the differential operators (the coefficients are divided by step(B)^k, where k is the order of the differential; since that is already accounted for in the case of non-uniform grids by scaling the coefficients, 1^k does not change the result):

julia> step(B1)

julia> step(B2)

The node locations and weights are shown in the figure below: Non-uniform grid

The basis functions become asymmetric in the non-uniform case:

julia> # Compute basis functions on a dense grid
       ξ = 10.0 .^ range(-3, stop=log10(25), length=1000);

julia> χ1 = B1[ξ, :];

julia> χ2 = B2[ξ, :];

Non-uniform basis functions

We then expand the function

\[f(x) = \sin(2\pi x/30)\exp(-4x/30)\]

on both grids and compare the error:

julia> f = x -> sin(2π*x/rmax)*exp(-4x/rmax)
#47 (generic function with 1 method)

julia> xx1 = axes(B1,1)

julia> c1 = B1 \ f.(xx1);

julia> xx2 = axes(B2,1)

julia> c2 = B2 \ f.(xx2);

julia> f1 = χ1*c1;

julia> f2 = χ2*c2;

julia> fe = f.(ξ);

Non-uniform basis functions