# how to make a nonlinear gird where grid points are not equally spaced?

I need to make a grid [0,1] with points that are concentrated close to the edges (close to 0 and 1) while the remaining points in the middle can be equally spaced. The reason for doing this is that I know the solution is very sensitive at the edges. Any ideas on how I can do this?

## 1 Answer

Presumably you are trying to use a finite difference method to solve a differential equation. The non-uniformity of the grid has an impact on accuracy. Hence, it is useful to include a parameter in the grid-generation algorithm that controls the rate at which the spacing increases away from the boundary.

There are many approaches for generating non-uniform grids (eg., exponential stretching, etc.) Searching on "numerical grid generation" should provide you with more information.

Here is a simple approach where the grid spacing increases geometrically. For the interval $[0,1/2]$ generate grid points using $x_0 = 0$ and for $k = 1,2,\ldots,n$,

$$x_k = \frac{\alpha\sum_{j=1}^k(1 + \alpha)^{j-1}}{2[(1 + \alpha)^n-1]}.$$

Simply reflect the points across $x = 1/2$ to generate the grid over $[1/2,1].$

Control the spacing and the difference $x_1-x_0$ by choosing appropriate values for the parameters $\alpha >0$ and $n$. Larger values for $\alpha$ will produce finer grids near the boundary.

• Thanks for your response. I was able to make the grid using the CDF of the beta distribution fairly easily (in about 10 lines). I ran this question by an applied math professor and she showed me a method that is based on the integral of tanh. The method is a bit complicated but is rather flexible in making the grid conform to a number of pre-specified restriction (eg. degree of concentration, symmetry, etc). Commented Feb 3, 2015 at 22:35
• @Cuedrah: You're welcome. Looks like you've covered some ground -- I was aware of the tanh method but have not looked at these things for a while. Out of curiosity, what are you trying to solve on the grid?
– RRL
Commented Feb 3, 2015 at 22:54