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I am working with the "difficult question of the representation of the fields" - cite by Peskin. With the approach of this paper as pdf, or this .ps anti-particle states get positive kinetic energy by the stress-energy tensor. The Klein paradox is solved by deriving the boundary condition by variation of the action. The polarized matrix elements of muon-pair creation by electron-positron scattering are computed directly from the fields - no need for trace technology.

The present version of the document was published August 6th 2020.

How to choose between several good options and maybe trying out new options has a mathematical solution. The Android App: Gittins Index, makes the solution available to everyone. Go here for an explanation of the use of the app and the underlying theory.

A mathematical fairy-tale about infinity. Only available in Danish.

"I am the greatest", a wonderfull story by Vagn Lundsgaard Hansen, professor emeritus of Mathematics at the Technical University of Denmark.

A football trainer asked me whether it was possible to design a certain kind of fair all-against-all competition, where the trainer talks with all of the players in turn, while the rest plays. It is - sometimes. Designs and theory.

In a class of 28 students, 7 groups of each 4 students prepare a presentation. In matrix groups each student must show the presentation for 6 from the other groups. This may be too many. Sometimes it is possible to work with smaller groups, still managing that everybody listens to each presentation once. In Danish, but everybody should understand the solutions.

The model was defined in order to study the dynamics of epidemics. It seems usefull as well in ecology and economics. The theory is thoroughly explained using only elementary mathematics. In the final chapter, applications to epidemics are discussed. Go to home page with the theory, and examples of computation of the parameters. Everything is in Danish.

On repetitive patterns in 2D, only in Danish. Download the notes.

Terese Marianne Olga Nielsen has made this overview of the tapestry groups.

I remain grateful to Terese for feedback and proposals for improving the notes.

This paper in Danish explains in great detail how M. Rejewski was able to break the code machine of the German Army back in 1933.

A diary in Danish covering 6 weeks after the operation. The SHA256 checksum of the document, should you wish to check its authenticity, is 09399c83851b78befd8f9db705c73048af457cb05f4c236d31abebecd2864a0c.

Extracts from letters received by the wellfare department of Wales:

- I have learned that you have branded my son illiterate. That is a dirty lie. We were married two weeks before he was born.
- According to your instructions I have given birth to a boy in the enclosed envelope. When do I get my money?
- If I don't get my money soon, I will have to live an immortal life.