John Fredsted

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17 years, 169 days

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These are Posts that have been published by John Fredsted

Yesterday evening I decided that I would like to make a list of links to the posts of mine which I consider most valuable. Today, however, I have realized that this is no easy task because the closest I can get to these posts is the "my recent posts" where only links to complete threads can be found. Therefore, would it be possible to get a list of ones own posts, each post with a pointer to its appropiate place in some thread? I guess that it boils down to some appropiate database query.
Inspired by the post Re: the physics package I decided to have a closer look at the function FeynmanDiagrams. As the Lagrangian I thought I might as well take the QED Lagrangian for a massless spinor field Q[i](X) coupled to an external electromagnetic field A[mu](X):
restart:
with(Physics):
Setup(advanced):
L_QED :=
   +Dagger(Q[i](X)) * Dgamma[4] * Dgamma[mu][i,j] * I * diff(Q[j](X),X[mu])
   +Dagger(Q[i](X)) * Dgamma[4] * Dgamma[mu][i,j] * e * A[mu](X) * (Q[j](X));
In trying to answer the second question in How to determine the order of an ODE? I have unsuccessfully tried to use select to filter out the appropiate parts of the differential equations. In these attempts I have hit upon a behaviour of select I do not understand: In my opinion each of the following two code lines (or at the very least the second line) should return the differential expression itself:
select(has,diff(y(t),t),y);
select(has,diff(y(t),t),y(t));
But they do not; each line returns diff(y(t)), where the variable t with respect to which is being differentiated is missing. Why? Is it just me having fundamentally misunderstood something?
For a long thread it is often a bit difficult to locate the new posts referred to in the Replies column of recent posts, especially if the thread has laid dormant for some period of time over which its detailed history has been forgotten. Therefore, could new posts be highlighted in some way to make them stand out?
This is a follow-up on one of the unresolved issues raised in a previous post of mine. In view of the fact that 1 + I is indeed of type complexcons, can anybody please explain why the following is reasonable:
restart:
assume(x::complexcons):
evalc(x + conjugate(x)),
evalc(x - conjugate(x));
restart:
x := 1 + I:
evalc(x + conjugate(x)),
evalc(x - conjugate(x));
                            2 x~, 0
                             2, 2 I
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