### TypeError: non-integral exponents not supported

Being new to Sage, I can't understand this error "TypeError: non-integral exponents not supported". This is raised by "find_root" in the following code snippet:

```
R.<x>=RR[]
n=1000
betas=[0.01,0.03,0.05,0.07,0.99,0.91]
#for beta in betas:
#for beta in srange(0.01,0.09,0.02):
for beta in srange(0.91,0.99,0.02):
f=x^n - 3*x^(n-1)+x^(n-2)+x^(n-3)-2*x^(n-beta*n-1)-3^(beta*n)
find_root(f, 1,4);
```

But what is strange for me is that if I replace the "for" line with one of the commented line, then it works well. So I think there must be some Sage concepts that I haven't been aware of.

Thanks for your help!