1 | initial version |

With Sage 7.5.1, you can use the exterior calculus on manifolds instead of `DifferentialForm`

; the advantage is that the simplification is automatic (no need to invoke `simplify_trig`

by hand): your example becomes:

```
sage: U = Manifold(2, 'U')
sage: X.<r, theta> = U.chart(r"r:(0,+oo) theta:(0,2*pi):\theta")
sage: x = U.scalar_field(r*cos(theta))
sage: y = U.scalar_field(r*sin(theta))
sage: a = x.differential().wedge(y.differential())
sage: a
2-form on the 2-dimensional differentiable manifold U
sage: a.display()
r dr/\dtheta
```

2 | No.2 Revision |

With Sage 7.5.1, you can use the exterior calculus on manifolds instead of `DifferentialForm`

; the advantage is that the simplification is automatic (no need to invoke `simplify_trig`

by hand): your example becomes:

```
sage: U = Manifold(2, 'U')
sage: X.<r, theta> = U.chart(r"r:(0,+oo) theta:(0,2*pi):\theta")
sage: x = U.scalar_field(r*cos(theta))
sage: y = U.scalar_field(r*sin(theta))
sage: a = x.differential().wedge(y.differential())
sage: a
2-form on the 2-dimensional differentiable manifold U
sage: a.display()
r dr/\dtheta
```

See http://doc.sagemath.org/html/en/reference/manifolds/sage/manifolds/differentiable/diff_form.html for more details.

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