Working with polynoms

Hi, everybody.

I wasn't successfull in finding the answer through the search. If the question was answered already, I'd be gratefull for a link.

The question itself:

if I've got an expression in brackets, I can expand it out of brackets, like so:

>> expand((s+4)*(s+0.5-0.2*i)*(s+0.5+0.2*i))
ans =s^3 + 5*s^2 + (429*s)/100 + 29/25

How can I make an opposite operation?

>> unexpand(s^3 + 5*s^2 + (429*s)/100 + 29/25)
ans =(s+4)*(s+0.5-0.2*i)*(s+0.5+0.2*i)

Or something like that.

I know, I can use the roots() function to calculate the expression myself, but I was wondering, if there is an easier way.

Thank you in advance for your answers.

Answer 1

[bym]

   syms s
  >> expand((s+4)*(s+0.5-0.2*i)*(s+0.5+0.2*i))

ans =

s^3 + 5*s^2 + (429*s)/100 + 29/25

>> factor(ans)

ans =

((s + 4)*(100*s^2 + 100*s + 29))/100

[edit]

ans =

s^3 + 5*s^2 + (429*s)/100 + 29/25

feval(symengine,'factor',ans,'C_')

ans =

(s + 4.0)*(s - 0.2*i + 0.5)*(0.2*i + s + 0.5)

[Boris]

That's a good way of rewriting the polynom, but it doesn't seem to work with complex roots. You still receive the 's' in the second power.

Is writing a code especially for this function the only solution?

[bym]

See my edit...

[Boris]

Excellent solution! this is exactly, what I need.

Thanks a million! :)