# Symbolics¶

PyWENO contains a symbolic module to help authors develop their own WENO methods and/or explore the basics of WENO methods. Below are a few quick examples demonstrating how the symbolic routines of PyWENO are used.

## Interpolating polynomials¶

First, let’s build some grid points and y-values:

>>> import sympy
>>> import pyweno
>>> (x0, x1, x2) = sympy.var('x0 x1 x2')
>>> (y0, y1, y2) = sympy.var('y0 y1 y2')


Then, the polynomial that interpolates the points (x0, y0), (x1, y1), and (x2, y2) is given by:

>>> p = pyweno.symbolic.polynomial_interpolator([x0, x1, x2], [y0, y1, y2])
>>> p
y0*(x - x1)*(x - x2)/((x0 - x1)*(x0 - x2)) + y1*(x - x0)*(x - x2)/((x1 - x0)*(x1 - x2)) + y2*(x - x0)*(x - x1)/((x2 - x0)*(x2 - x1))


and is a function of the SymPy variable x. For example:

>>> x = sympy.var('x')
>>> p.subs(x, x2)
y2


The polynomial that interpolates the primitive function of $$f$$ such that

$f(x_i) = \sum_j y_j (x_{j+1} - x_{j})$

is given by:

>>> P = pyweno.symbolic.primitive_polynomial_interpolator([x0, x1, x2], [y1, y2])
>>> P
y1*(x - x0)*(x - x2)/(x1 - x2) + (x - x0)*(x - x1)*(y1*(x1 - x0) + y2*(x2 - x1))/((x2 - x0)*(x2 - x1))


and is also a function of the SymPy variable x. For example:

>>> P.subs(x, x1)
y1*(x1 - x0)


For uniform grids, one could define the grid points by:

>>> (x, dx) = sympy.var('x dx')
>>> xs = [ dx, 2*dx, 3*dx ]
>>> p = pyweno.symbolic.polynomial_interpolator(xs, [y0, y1, y2])
>>> p
y0*(x - 3*dx)*(x - 2*dx)/(2*dx**2) + y2*(x - dx)*(x - 2*dx)/(2*dx**2) - y1*(x - dx)*(x - 3*dx)/dx**2


## Reconstruction coefficients¶

Hereafter we assume that the grid is uniform. Furthermore, to specify a point within a cell, the interval [-1, 1] is used as a reference.

The reconstruction coefficients for a 5th (=2k-1 where k=3) order WENO scheme corresponding to the reconstruction point at the left side ($$\xi$$ = -1) of each grid cell are given by:

>>> c = pyweno.symbolic.reconstruction_coefficients(k=3, xi=[ -1 ])
>>> c
{'k': 3,
'n': 1,
(0, 0, 0): 11/6,
(0, 0, 1): -7/6,
(0, 0, 2): 1/3,
(0, 1, 0): 1/3,
(0, 1, 1): 5/6,
(0, 1, 2): -1/6,
(0, 2, 0): -1/6,
(0, 2, 1): 5/6,
(0, 2, 2): 1/3


Note that the return value c is a dictionary of SymPy objects, indexed according to c[l,r,j] where l is the index of the reconstruction point and r is the left-shift of the stencil.

Recall that the reconstruction coefficients $$c$$ are used to reconstruct the original (unknown) function $$f$$ at each point $$\xi_l$$ in xi according to

$f^r(\xi^l) \approx \sum_{j=0}^{k-1} c^l_{r,j} \, \bar{f}_{i-r+j}$

for each $$l$$ from 0 to len(xi), where $$\bar{f}_{i-r+j}$$ is the cell average of $$f$$ in the cell $$i-r+j$$.

## Optimal weights¶

The optimal weights for a 5th (=2k-1 where k=3) order WENO scheme corresponding to the reconstruction point at the left side of each grid cell are given by:

>>> w = pyweno.symbolic.optimal_weights(3, [ -1 ])
>>> w
({'k': 3, 'n': 1, (0, 0): 1/10, (0, 1): 3/5, (0, 2): 3/10}, {0: False, 'n': 1})


Note that the return value w is a tuple of dictionaries of SymPy objects. The first dictionary contains the weights, and is indexed according to w[l,r]. the second dictionary contains boolean values determining if the weights are split (negative).

Recall that the optimal weights are used to obtain an optimally high-order reconstruction of the original function $$f$$ given the low-order reconstructions $$f^r(\xi^l)$$ according to

$f(\xi^l) \approx \sum_{r=0}^{k-1} \varpi^{l,r} f^r(\xi_l).$

## Smoothness coefficients¶

The Jiang-Shu smoothness coefficients for a 5th (=2k-1 where k=3) order WENO scheme are given by:

>>> beta = pyweno.symbolic.jiang_shu_smoothness_coefficients(3)


The return value beta is a dictionary of SymPy objects, and is indexed according to beta[r,m,n] (see the reference documentation for details).

Recall that the smoothness coefficients beta[r, m, n] are used to compute the non-linear weights $$\omega^{l,r}$$ (used in place of $$\varpi^{l,r}$$ in non-smooth regions) according to

$\omega^{l,r} = \frac{\alpha^{l,r}}{\alpha^{l,0} + \cdots + \alpha_{l,k-1}}$

where

$\alpha^{l,r} = \frac{\varpi^{l,r}}{(\epsilon + \sigma^r)^p}$

and

$\sigma^r = \sum_{m=1}^{2k-1} \sum_{n=1}^{2k-1} \beta_{r,m,n}\, \overline{f}_{i-k+m}\, \overline{f}_{i-k+n}.$