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

1. Apr 18, 2012

### hiroman

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?

Thanks!

2. Apr 18, 2012

### SteveL27

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.

3. Apr 18, 2012

### hiroman

Many thanks for the enlightenment, Steve! Cheers.