# Piecewise Polynomial Quadrature Formula

I've recently discovered a family of quadrature rules that I don't recognize and may be novel. It is an interpolation based quadrature that uses local piecewise polynomial interpolation. It is distinct from splines and from Newton-Cotes type quadratures. It works on arbitrary nodes. For equally spaced nodes it appears to be the trapezoidal rule, but with corrections near the boundary, similar to Gregory's method though it appears to be distinct.

# Interpolation

We will construct a piecewise interpolant for a function $f: [a,b] \to \RR$
which we will call the *local polynomial interpolant* or just *local interpolant* for short.
Let $\DD \subset \PP([a, b])$ be a partition of $[a, b]$.
Let $\Xi = \{x_i\}_{i=1}^N \subset [a, b]$ which we will call *interpolation nodes*.
For each subdomain $D \in \DD$ we will associate a stencil.
More precisely, we will define the mapping $S: \DD \to \PP(\Xi)$ such that $S(D) \subset \Xi$ is non-empty for every $D \in \DD$,
and we will refer to $S(D)$ as the stencil associated with the element $D$.
Let $p_D$ be the polynomial that interpolates $f$ at the nodes $S(D)$.
Our piecewise polynomial interpolant is then given by
$$
s(x) = \begin{cases}
p_D(x), & \text{ for } x \in D.
\end{cases}
$$

This interpolation is parameterized by the set of interpolation points $\Xi$, the partition $\DD$, and the associated stencils given by the mapping $S: \DD \to \PP(\Xi)$. In principle these are all arbitrary, but there should be some reasonable restrictions imposed in order for the interpolant to be a good approximation to the function.

### Parameter selection

A natural partion is given by the interpolation nodes (assuming they are ordered)
$$
\DD = \{ [a, x_1], [x_1, x_2], [x_2, x_3], ..., [x_{n-1}, x_n], [x_n, b] \}
$$
and possibly removing the first and last intervals if $a = x_1$ and $x_n = b$ respectively.
We can then select stencils by choosing an *order parameter* $k$, and defining $S([x_i, x_{i+1}])$
to be the $k$ closest points in $\Xi$ to the interval.
If the points are well distributed then the interval $[x_i, x_{i+1}]$ will be near the
center of the stencil, except near the boundary where the stencil will become one-sided.

# Equally spaced nodes

In the special case of equally spaced points (including endpoints) and $k = 2$ this gives the piecewise linear interpolant and thus will reproduce the trapezoidal rule. Though this procedure will yield a piecewise polynomial interpolant, we do not enforce any smoothness and thus this is different than spline interpolation as demonstrated by the figure below.

In fact, if the boundaries of the partition do not coincide with interpolation nodes then the local interpolant need not even be continuous, though we do not explore this case in detail here.

To understand the space of interpolants, it is helpful to look at a cardinal basis for this space. We can construct basis functions by setting the interpolation values to zero for all points except one. Doing so yields the basis functions in the figure below (also compared to corresponding cubic spline basis functions).

Here we make several observations. First, we see that (with the exception of the first two functions near the boundary) that the basis functions are not smooth. We also see that their support is smaller than the entire domain. Specifically, any given interpolation point will only be contained in the stencil of $2k$ intervals and thus will only affect the resulting interpolant on those intervals. It's difficult to see, but cubic splines (in general) are non-zero over the entire interval. Thus changing an interpolation value will change the spline interpolant over each interval.

Since quadrature weights are given by analytically integrating these cardinal functions we also see that the weights for the interior nodes must be identical. Since this interpolation scheme reproduces polynomials, and in particular constants, the quadrature weights for interior nodes must be $1/h$ where $h$ is the mesh spacing.

# Family of Quadrature Formulae

For equally spaced nodes, choosing an even order parameter $k$ leads to the following family of quadrature formulae, where we only give the weights near the left boundary as the interior weights are $\frac{1}{h}$ and the weights at the right boundary are reflections of the left boundary.

For $\mathcal{O}(h^2)$. $\frac{1}{2h} \big[ 1 \ 2 \ 2 \ 2 \ 2 ...$

For $\mathcal{O}(h^4)$: $\frac{1}{24h} \big[ 8 \ 31 \ 20 \ 25 \ 24 \ 24 \ 24 ...$

For $\mathcal{O}(h^6)$: $\frac{1}{1440h} \big[ 459 \ 1982 \ 944 \ 1746 \ 1333 \ 1456 \ 1440 \ 1440 \ 1440 ...$

We can present these weights in a standardized format: choose a grid to be the positive integers (so $h=1$) and only report the weights that differ from 1.

$\mathcal{O}(2)$ | $\frac{1}{2}$ | |||||||||||||||

$\mathcal{O}(4)$ | $\frac{1}{3}$ | $\frac{31}{24}$ | $\frac{5}{6}$ | $\frac{25}{24}$ | ||||||||||||

$\mathcal{O}(6)$ | $\frac{51}{160}$ | $\frac{991}{720}$ | $\frac{59}{90}$ | $\frac{97}{80}$ | $\frac{1333}{1440}$ | $\frac{91}{90}$ | ||||||||||

$\mathcal{O}(8)$ | $\frac{278}{945}$ | $\frac{185153}{120960}$ | $\frac{3667}{15120}$ | $\frac{8167}{4480}$ | $\frac{733}{1890}$ | $\frac{156451}{120960}$ | $\frac{2777}{3024}$ | $\frac{905}{896}$ | ||||||||

$\mathcal{O}(10)$ | $\frac{81385}{290304}$ | $\frac{5982811}{3628800}$ | $- \frac{105103}{518400}$ | $\frac{3384373}{1209600}$ | $- \frac{27673}{28350}$ | $\frac{371081}{145152}$ | $\frac{175523}{1209600}$ | $\frac{4758181}{3628800}$ | $\frac{6767167}{7257600}$ | $\frac{14269}{14175}$ | ||||||

$\mathcal{O}(12)$ | $\frac{1657}{6160}$ | $\frac{1693103359}{958003200}$ | $- \frac{183182141}{239500800}$ | $\frac{155823623}{35481600}$ | $- \frac{52948363}{13305600}$ | $\frac{41542229}{6386688}$ | $- \frac{54633}{15400}$ | $\frac{601537459}{159667200}$ | $- \frac{2733413}{13305600}$ | $\frac{48112633}{35481600}$ | $\frac{44838553}{47900160}$ | $\frac{38522153}{38320128}$ | ||||

$\mathcal{O}(14)$ | $\frac{27770156197}{106748928000}$ | $\frac{4910982739693}{2615348736000}$ | $- \frac{1830414679453}{1307674368000}$ | $\frac{17308443934079}{2615348736000}$ | $- \frac{3239871500473}{348713164800}$ | $\frac{6802893055867}{435891456000}$ | $- \frac{105610027}{7007000}$ | $\frac{130582029653}{8895744000}$ | $- \frac{13824839392867}{1743565824000}$ | $\frac{2819830208717}{523069747200}$ | $- \frac{752403440483}{1307674368000}$ | $\frac{3634010752403}{2615348736000}$ | $\frac{4920175305323}{5230697472000}$ | $\frac{28145907}{28028000}$ | ||

$\mathcal{O}(16)$ | $\frac{69181108}{273648375}$ | $\frac{124527838997953}{62768369664000}$ | $- \frac{8301345801121}{3923023104000}$ | $\frac{602923312676921}{62768369664000}$ | $- \frac{1596315823547}{89159616000}$ | $\frac{2120764633122901}{62768369664000}$ | $- \frac{172974549513301}{3923023104000}$ | $\frac{21497071030031}{426995712000}$ | $- \frac{53570696141}{1277025750}$ | $\frac{1918959527598691}{62768369664000}$ | $- \frac{58518753821611}{3923023104000}$ | $\frac{474505422337963}{62768369664000}$ | $- \frac{980645013239}{980755776000}$ | $\frac{8132582533301}{5706215424000}$ | $\frac{528870628631}{560431872000}$ | $\frac{1285469654383}{1280987136000}$ |

These quadrature formulae are similar to the Gregory method in that they appear to be boundary corrections to the trapezoidal rule. The number of terms in a correction of a given order appears to be larger however. Also, choosing an odd order parameter makes stencil selection ambiguous and ruins the symmetry properties, which doesn't correspond Gregory's method.

# Convergence

These order parameters appear to coincide with convergence order as the number of points grows. Below we show several numerical experiments approximating the order of convergence for several test functions.

# Advantages

The principle advantage to this method is flexibility. One can easily change the location of the nodes and derive quadrature weights. Additionally, one can easily change the choice of basis functions. Indeed, I discovered this method when working on Local Radial Basis Function Interpolation, which has the advantage of working well in arbitrary dimensions.

Notably, one could enforce more constraints and increase the degree of polynomial. One can enforce smoothness conditions to get splines with higher order convergence. One can enforce Hermite interpolation conditions (possibly only at the boundary) to reduce the number of end corrections. I suspect that something like this can be used to reproduce the Gregory weights.

# Future Directions

It would be interesting to see if this could be the foundation of an adaptive quadrature method. Since the weights are determined by the location of only a few nearby nodes, selectively adapting the weights could allow for minimal function evaluation for a high-order adaptive method.

Perhaps there is potential for maintaining high-order convergence for piecewise smooth functions if the locations of the knots are known in advance by enforcing that only a certain number of stencil nodes are allowed to cross these boundaries. For example, if you enforce that no nodes may cross the boundary, this is simply domain decomposition and of course will work for piecewise continuous functions.

In the table above, we see that some of the weights become negative for orders 10 and above. Perhaps there is a way to avoid this by clustering nodes near the boundary. This would increase the number of end corrections since the stencil-translation argument wouldn't hold until far away from the non-equally spaced nodes, but perhaps it could avoid Runge's phenomenon, which I assume is the underling cause.

It would be interesting to apply this to infinite domains with appropriate weight kernels, as in Gauss-Hermite or Gauss-Legendre interpolation. One could potentially get more accuracy by adding more points without sacrificing stability.