Great thanks very much - I copied from the example - I guess I should have 
checked the manual - I wonder if there is some way of making sure the 
examples are always up to date.

On Wednesday 7 February 2024 at 12:43:47 UTC Vincent Delecroix wrote:

> The error you get indicates that "exterior_der" is not a valid
> operation. Maybe you want "exterior_derivative()"?
>
> On Wed, 7 Feb 2024 at 13:16, Dominic Steinitz <idontge...@gmail.com> 
> wrote:
> >
> > I am in the examples folder trying to run
> >
> > Electromagnetism in Minkowski spacetime [ipynb] (Electromagnetic field 
> 2-form from E and B, Maxwell equations, conserved current, Lorentz force, 
> Poynting vector)
> >
> >
> > but I get the error
> >
> > AttributeError: 'DiffFormFreeModule_with_category.element_class' object 
> has no attribute 'exterior_der'
> > Here's a MWE
> > M = Manifold(4, 'M', latex_name=r'\mathcal{M}', structure='Lorentzian') 
> BL.<t,x,y,z> = M.chart(r"t x y z")
> > var('E_x E_y E_z')
> > dt, dx, dy, dz = BL.coframe()[:]
> > E = E_x * dx + E_y * dy + E_z * dzE.set_name('E')
> > (E.wedge(BL.coframe()[0])).exterior_der()
> >
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>

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