# Units in number fields.

I want to get the expression of a unit in a number field in terms of the base by means of `U.log()`

.
Here is my code:

```
K.<z> = CyclotomicField(21)
U = UnitGroup(K)
U.fundamental_units()
a=(z^3+z^6+z^12)/2+1 # a is (3 + i sqrt[7])/4 so it is a unit in K
U.log(a)
```

I get the error

```
ValueError: 1/2*z^11 - 1/2*z^9 + 1/2*z^8 + 1/2*z^4 + 1/2*z + 1/2 is not a unit
```

But it is a unit. It is true that `a = 1/2*z^11 - 1/2*z^9 + 1/2*z^8 + 1/2*z^4 + 1/2*z + 1/2`

.

What I'm doing wrong ?

@tmonteil Thanks for the edits.