1 | initial version |

Avoid the symbolic ring whenever you can, and work in the appropriate structures for the task at hand.

Manipulate polynomials and rational fractions in the corresponding structures. Define

```
sage: R.<z> = PolynomialRing(QQ)
sage: F = FractionField(R)
sage: S = PowerSeriesRing(QQ,'z')
```

and check the result:

```
sage: R
Univariate Polynomial Ring in z over Rational Field
sage: F
Fraction Field of Univariate Polynomial Ring in z over Rational Field
sage: S
Power Series Ring in z over Rational Field
```

Define your polynomials and rational fractions.

```
sage: a = 2 - 3*z + z^2
sage: b = z
sage: f = a / b
```

Check what you get.

```
sage: a
z^2 - 3*z + 2
sage: b
z
sage: f
(z^2 - 3*z + 2)/z
```

Check out where a, b, f live.

```
sage: a.parent()
Univariate Polynomial Ring in z over Rational Field
sage: b.parent()
Univariate Polynomial Ring in z over Rational Field
sage: f.parent()
Fraction Field of Univariate Polynomial Ring in z over Rational Field
```

Want the Taylor series at 1 of f, and its inverse under composition.

First shift the function to move to the point 1.

```
sage: g = f(1+z)
sage: g
(z^2 - z)/(z + 1)
```

Move to the ring of power series.

```
sage: gg = S(g)
```

Check the first few terms.

```
sage: gg + O(z^8)
-z + 2*z^2 - 2*z^3 + 2*z^4 - 2*z^5 + 2*z^6 - 2*z^7 + O(z^8)
```

The constant term is zero and the coefficient of z is nonzero: we can compute the composition inverse.

```
sage: hh = gg.reversion()
```

Examine the first few terms.

```
sage: hh + O(z^8)
-z + 2*z^2 - 6*z^3 + 22*z^4 - 90*z^5 + 394*z^6 - 1806*z^7 + O(z^8)
```

2 | No.2 Revision |

Avoid the symbolic ring whenever you can, and work in the appropriate structures for the task at hand.

Manipulate polynomials and rational fractions in the corresponding structures. Define

```
sage: R.<z> = PolynomialRing(QQ)
sage: F = FractionField(R)
sage: S = PowerSeriesRing(QQ,'z')
```

and check the result:

```
sage: R
Univariate Polynomial Ring in z over Rational Field
sage: F
Fraction Field of Univariate Polynomial Ring in z over Rational Field
sage: S
Power Series Ring in z over Rational Field
```

Define your polynomials and rational fractions.

```
sage: a = 2 - 3*z + z^2
sage: b = z
sage: f = a / b
```

Check what you get.

```
sage: a
z^2 - 3*z + 2
sage: b
z
sage: f
(z^2 - 3*z + 2)/z
```

Check out where a, b, f live.

```
sage: a.parent()
Univariate Polynomial Ring in z over Rational Field
sage: b.parent()
Univariate Polynomial Ring in z over Rational Field
sage: f.parent()
Fraction Field of Univariate Polynomial Ring in z over Rational Field
```

Want the Taylor series at 1 of f, and its inverse under composition.

First shift the function to move to the point 1.

```
sage: g = f(1+z)
sage: g
(z^2 - z)/(z + 1)
```

Move to the ring of power series.

```
sage: gg = S(g)
```

Check the first few terms.

```
sage: gg + O(z^8)
-z + 2*z^2 - 2*z^3 + 2*z^4 - 2*z^5 + 2*z^6 - 2*z^7 + O(z^8)
```

The constant term is zero and the coefficient of z is nonzero: we can compute the composition inverse.

```
sage: hh = gg.reversion()
```

Examine the first few terms.

```
sage: hh + O(z^8)
-z + 2*z^2 - 6*z^3 + 22*z^4 - 90*z^5 + 394*z^6 - 1806*z^7 + O(z^8)
```

Check that hh is the composition inverse of gg:

```
sage: hh(gg)
z + O(z^21)
sage: gg(hh)
z + O(z^21)
```

3 | No.3 Revision |

Avoid the symbolic ring whenever you can, and work in the appropriate structures for the task at hand.

Manipulate polynomials and rational fractions in the corresponding structures. Define

```
sage: R.<z> = PolynomialRing(QQ)
sage: F = FractionField(R)
sage: S = PowerSeriesRing(QQ,'z')
```

and check the result:

```
sage: R
Univariate Polynomial Ring in z over Rational Field
sage: F
Fraction Field of Univariate Polynomial Ring in z over Rational Field
sage: S
Power Series Ring in z over Rational Field
```

Define your polynomials and rational fractions.

```
sage: a = 2 - 3*z + z^2
sage: b = z
sage: f = a / b
```

Check what you get.

```
sage: a
z^2 - 3*z + 2
sage: b
z
sage: f
(z^2 - 3*z + 2)/z
```

Check out where a, b, f live.

```
sage: a.parent()
Univariate Polynomial Ring in z over Rational Field
sage: b.parent()
Univariate Polynomial Ring in z over Rational Field
sage: f.parent()
Fraction Field of Univariate Polynomial Ring in z over Rational Field
```

Want the Taylor series at 1 of f, and its inverse under composition.

First shift the function to move to the point 1.

```
sage: g = f(1+z)
sage: g
(z^2 - z)/(z + 1)
```

Move to the ring of power series.

```
sage: gg = S(g)
```

Check the first few terms.

```
sage: gg + O(z^8)
-z + 2*z^2 - 2*z^3 + 2*z^4 - 2*z^5 + 2*z^6 - 2*z^7 + O(z^8)
```

The constant term is zero and the coefficient of z is nonzero: we can compute the composition inverse.

```
sage: hh = gg.reversion()
```

Examine the first few terms.

```
sage: hh + O(z^8)
-z + 2*z^2 - 6*z^3 + 22*z^4 - 90*z^5 + 394*z^6 - 1806*z^7 + O(z^8)
```

Check that hh is the composition inverse of gg:

```
sage: hh(gg)
z + O(z^21)
sage: gg(hh)
z + O(z^21)
```

EDIT (to answer your follow-up question).

If you have a rational fraction `g`

, then `gnum = g.numerator()`

and `gden = g.denominator()`

will get you the polynomials for the numerator and the denominator of `g`

. You can check where they live by `gnum.parent()`

and `gden.parent()`

. If you need them to be in `R`

, `F`

or `S`

, just do `R(gnum)`

, `F(gnum)`

, `S(gnum)`

, etc.

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