# Numerical integration and plot failing

I have a function which can be efficiently calculated on some interval on the real line and if I feed Sage with concrete arguments, Sage gives concrete numerical values. Nevertheless, if I try to integrate it numerically (or plot it), something goes terribly wrong and Sage is not able to calculate the values.

In my case I get the error message

```
negative number cannot be raised to a fractional power
```

The function itself is a solution to the cubic equation so it involves square root and a cubic root.

is it some bug in sage and if there is some workaround?

MINIMAL NON_WORKING EXAMPLE

```
y = var('y')
z = var('z');
cauchy = y.substitute(solve(z*y^3 +y^2 - 2*z*y+2, y)[0])
def F(u):
return arg(cauchy).substitute(z=u).n()
numerical_integral(F, 1, 2)
```

Indeed :

Therefore, this function

canbe evaluated. But :... not in

`plot`

. Trying to plot symbolically yelds :A bug, IMHO