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

Cubic polynomial function with 3 real roots; one at infinity?

  1. Apr 18, 2012 #1
    Is it possible to have a cubic polynomial (ax^3+bx^2+cx+d) which has three REAL roots, with one of them being +/- infinity?

    If there is, can you give an example?

  2. jcsd
  3. Apr 18, 2012 #2
    It doesn't make literal sense for any function whose domain is the real numbers to have a root at infinity, since infinity's not an element of the domain.

    But we could still ask if there's a 3rd degree polynomial such that the limit as x->inf is 0. The answer's no, and it's easy to see. As x -> +inf, x^3 goes to +inf. Since the x^3 term eventually dominates the rest of the terms, the polynomial goes to +inf.

    Likewise, as x -> -inf, the function must go to -inf. So it's not possible for the polynomial to go to zero at +/- infinity. This reasoning goes through for any odd-degree polynomial. For even-degree polynomials, the limits at +/- infinity are both infinity.
  4. Apr 18, 2012 #3
    Many thanks for the enlightenment, Steve! Cheers.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook