In Matlab conv with syms argument

1
votes

I have an equation: w = (t-x0)*(t-x1). I want to solve it with the conv function(conv((t-x0),(t-x1))), but its arguments are syms which are t,x0 and x1. I get an error that is

Undefined function 'conv2' for input arguments of type 'sym'.

How do I solve its error ? I also want the result to be a polynomial, because I should integrate with polyint.

For example:

w = (t-x0)*(t-x1) --> w = t^2 - t*(x0+x1) + x0*x1 --> w=[ 1   -x0-x1   x0*x1 ]

polyint(w) -->  w= t^3/3 -t^2/2*(x0+x1) + t*x0*x1 --> w=[ 1/3  -1/2*(x0+x1)  x0*x1  0 ]
2
matlabsymbolic-math
I don't understand this question. What made you think conv was appropriate in the first place? - nkjt
I have though, it would be conv func. How do you think I can do what have showed above for solution . - Gokhan Sahin
If you were just looking for t^3/3 + (- x0/2 - x1/2)*t^2 + x0*x1*t as an answer, then why did you say you wanted w=[ 1/3 -1/2*(x0+x1) x0*x1 0 ] ? If all you want is t^3/3 + (- x0/2 - x1/2)*t^2 + x0*x1*t then it is one line in Matlab. I will update my answer to show this as well. - Nasser

2 Answers

2
votes

I don't think Matlab yet has a default function for symbolic convolution (although Mr. Moler offers a shim here), but that's not a big deal in this instance since as it is mentioned: "If [the inputs] are vectors of polynomial coefficients, convolving them is equivalent to multiplying the two polynomials." So we can use straight up multiplication.

>> syms t x0 x1
>> w = (t-x0)*(t-x1);
>> p = fliplr(coeffs(w,t))
    p =
    [ 1, - x0 - x1, x0*x1]
>> pint = polyint(p)
    pint =
    [ 1/3, - x0/2 - x1/2, x0*x1, 0]
>> wint = poly2sym(pint,t)
    wint =
    t^3/3 + (- x0/2 - x1/2)*t^2 + x0*x1*t

Note that I flipped the order from coeffs since the order is the reverse of the poly* family of functions

2
votes
clear
syms t x0 x1;
r = int((t-x0)*(t-x1),t);
c = evalin(symengine,sprintf('coeff(%s, t)',char(r)));
c0= evalin(symengine,sprintf('coeff(%s, t,0)',char(r)));
if c0==0
   c=[c 0];
end

gives

  [ 1/3, - x0/2 - x1/2, x0*x1, 0]

Update:

It looks OP just wants:

syms t x0 x1;
r=int((t-x0)*(t-x1),t)

gives

t^3/3 + (- x0/2 - x1/2)*t^2 + x0*x1*t