**EDIT**: The comment of @BGS indicates that the question migh be understood as follows : given an element of a cyclotomic field, how to recover it as a polynomial of the generator, so that we can "recast" the generator into a generator of another cyclotomic field. Here is a simple solution, involving the `.polynomial()`

method of cyclotomic field elements.

Finding the polynomial:

```
sage: C7 = CyclotomicField(7)
sage: z7 = C7.gen()
sage: a7 = z7 + 3*z7^2 + 1
sage: a7.polynomial()
x^3 + 3*x^2 + 1
sage: a7.polynomial().parent()
Univariate Polynomial Ring in x over Rational Field
```

Recasting into another cyclotomic field:

```
sage: C5 = CyclotomicField(5)
sage: z5 = C5.gen()
sage: a5 = a7.polynomial()(z5)
sage: a5
zeta5^3 + 3*zeta5^2 + 1
```

That said, the polynomial is not uniquely determined (unless you need the minimal one) and the recasting will depend on the polynomial, since for example:

```
sage: z5^5 + z5 == z5 + 1
True
sage: z7^5 + z7 == z7 + 1
False
```

**PREVIOUS ANSWER** (answering a question about embedding cyclotomic field generators into the algebraic field)

The problem is that the cyclotomic field is not well embedded into the complex plane (more precisely into the field of complex algebraic numbers), see my answer of ask question 25822 for more details about this.

```
sage: CC(z3)
-0.500000000000000 + 0.866025403784439*I
sage: C3 = CyclotomicField(3)
sage: z3 = C3.gen()
sage: z3
zeta3
sage: CC(z3)
-0.500000000000000 + 0.866025403784439*I
```

but

```
sage: QQbar(z3)
TypeError: Illegal initializer for algebraic number
```

Which is a pity. Let me propose the following fix:

```
sage: def repair(z):
....: F = z.parent()
....: for f in F.embeddings(QQbar):
....: if CLF(f.im_gens()[0]) - CLF(z) < 1e-14:
....: return f(z)
```

The function `repair`

put an generator of some cyclotomic field into the algebraic field `QQbar`

(if `n`

is not too big, so that two generators are at distance larger than `2e-14`

, but you can adapt the bound if needed).

So you can do:

```
sage: C3 = CyclotomicField(3)
sage: z3 = C3.gen()
sage: Z3 = repair(z3)
sage: Z3
-0.50000000000000000? - 0.866025403784439?*I
sage: Z3.parent()
Algebraic Field
sage: C4 = CyclotomicField(4)
sage: z4 = C4.gen()
sage: Z4 = repair(z4)
sage: Z4
-1*I
sage: Z4.parent()
Algebraic Field
```

Then you can do:

```
sage: Z3 + Z4
-0.50000000000000000? - 1.866025403784439?*I
sage: (Z3^2 + 2*Z3 + 1) + (Z4^3 + 7)
6.5000000000000000? + 0.1339745962155614?*I
sage: ((Z3^2 + 2*Z3 + 1) + (Z4^3 + 7)).minpoly()
x^4 - 26*x^3 + 257*x^2 - 1144*x + 1933
```