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