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Polynomials (2) and Rational Functions

We have learnt that polynomials may be represented by the vector of their coefficient. Using a symbolic variable x we will now create a symbolic polynomial p. Conversely, we can extract the coefficients from a symbolic polynomial using the function coeff(p, x, exponent). The command allroots(p) returns the zeros.

>> a=[3 2 5 7 4];   % coefficients
>> syms x
>> y=polyval(a,x)   % symbolic polynomial
y = 3*x^4+2*x^3+5*x^2+7*x+4   
>> coeff(y,x,3)     % get one coefficient
ans = 2     
>> b=coeff(y,x,4:-1:0)  % or all at once
b = [ 3  2  5  7  4 ]
>> allroots(y)      % same as roots(a)
ans = [ 0.363-1.374i  0.363+1.374i 
          -0.697-0.418i  -0.697+0.418i ]

Up to this point there is little advantage of using symbolic calculations, it is just another way of specifying a problem. The main benefit of symbolic calculations emerges when we are dealing with more than one symbolic variable, or, meaning essentially the same, when our polynomial has nonconstant coefficients. This case can be treated efficiently only with symbolic variables. Notice in the example below how the polynomial y is automatically multiplied through, and brought into a canonical form. In this form the symbolic variables are sorted alphabetically, i.e. z is main variable compared to x. The coefficients can be calculated for each variable separately.

>> syms x,z
>> y=(x-3)*(x-1)*(z-2)*(z+1)
y = (x^2-4*x+3)*z^2+(-x^2+4*x-3)*z+(-2*x^2+8*x-6)
>> coeff(y,x,2)
ans = z^2-z-2
>> coeff(y,z,2)
ans = x^2-4*x+3



Subsections
next up previous contents
Next: Roots Up: Working with Jasymca Previous: Symbolic Variables
Helmut Dersch
2009-03-15