Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about the permutations of roots as polynomial coefficients

  1. Dec 20, 2011 #1
    Ok, so obviously, given some polynomial P(x) of degree r, it has r roots in the complex numbers by the FTOA, and if these roots are u_1, u_2,... it can be written as

    P(x) = (x - {u_1})(x - {u_2})(x - {u_3}) \cdots \\
    P(x) = {x^r} - ({u_1} + {u_2} + {u_3} + \ldots ){x^{r - 1}} + ({u_1}{u_2} + {u_1}{u_3} + {u_2}{u_3}+ \ldots){x^{r - 2}} - ({u_1}{u_2}{u_3} + \ldots ){x^{r - 3}} + \ldots

    Obviously the coefficient of the r-n power is the sum over the permutations of the r roots taken n at a time.

    My question is:

    Is there a shorthand notation for referring to this in an equation, i.e. a combinatorial expression in terms of the discrete set of roots?

    Obviously when talking about just plain numbers we can refer to the standard nCr and nPr formulas, but in this case we must view the set of roots as a discrete set of objects rather than numbers, hence my dilemma.
    Last edited: Dec 20, 2011
  2. jcsd
  3. Dec 20, 2011 #2
  4. Dec 20, 2011 #3
    That is exactly what I was looking for.

    Thank you
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook