In this post I will show you how to calculate and plot the Chebyshev Polynomials of the second kind using a C Program and Gnuplot.
We will be using the following information:
and the recurrence relation:
where, n starts from 1.
Modifying the recurrence relation a little, so that n starts from 2, we get:
We will create a program that calculates the values of the Chebyshev Polynomials at various x values and for different n and store these values in a txt file. Then just plot it using Gnuplot.
We will create two functions called ‘U0’ and ‘U1’, that contain the definition of respectively.
Then we will create a function ‘Un’ that will use the first two functions and recursion to find the value of Chebyshev Polynomials for different x,n.
PROGRAM:
/****************************************** ***********Chebyshev Polynomials*********** ******************************************/ #include<stdio.h> double U0(double x){ return 1; } double U1(double x){ return 2*x; } //General form of Chebyshev polynomial of second for a given value of n and x double Un(double x, int n){ if(n==0){ return U0(x); } else if(n==1){ return U1(x); } else{ //using the recurrence relation return 2*x*Un(x,n-1)-Un(x,n-2); } } main(){ double x; FILE *fp=NULL; fp=fopen("cheby.txt","w"); //Write down the values to a file for(x=-1;x<=1;x=x+0.01){ fprintf(fp,"%lf\t%lf\t%lf\t%lf\t%lf\t%lf\n",x,Un(x,0),Un(x,1),Un(x,2),Un(x,3),Un(x,4)); } }
When you run the above C, it will generate a file called ‘cheby.txt’ which would contain 6 columns of data-points.
The first column contains the ‘x’ values and the rest of them are for
These can be easily plotted using Gnuplot by using the following commands:
GnuPlot Command:
->set xlabel 'x'
->plot 'cheby.txt' u 1:2 w l t "U0(x)", '' u 1:3 w l t "U1(x)", '' u 1:4 w l t "U2(x)", '' u 1:5 w l t "U3(x)", '' u 1:6 w l t "U4(x)"
OUTPUT(Gnuplot):
References:
https://en.wikipedia.org/wiki/Chebyshev_polynomials
http://mathworld.wolfram.com/ChebyshevPolynomialoftheSecondKind.html
Ph.D. researcher at Friedrich-Schiller University Jena, Germany. I’m a physicist specializing in computational material science. I write efficient codes for simulating light-matter interactions at atomic scales. I like to develop Physics, DFT, and Machine Learning related apps and software from time to time. Can code in most of the popular languages. I like to share my knowledge in Physics and applications using this Blog and a YouTube channel.