Fix sign_changes_at

Author: hamarb123

src/algebraic_numbers.rs

@@ -200,11 +200,13 @@ fn sign_changes_at(
             ValueOrInfinity::Zero => Sign::new(&polynomial.coefficient(0)),
             ValueOrInfinity::NegativeInfinity => {
                 let degree = polynomial.degree().unwrap_or(0);
-                if degree == 0 || degree.is_odd() {
-                    Sign::new(&polynomial.highest_power_coefficient())
-                } else {
-                    Some(Sign::Positive)
+                let s = Sign::new(&polynomial.highest_power_coefficient());
+                if degree.is_odd()
+                {
+                    if let Some(sign) = s { Some(-sign) }
+                    else { None }
                 }
+                else { s }
             }
         };
         if let Some(sign) = sign {