Differentiable manifold Updated 2025-07-16
TODO find a concrete numerical example of doing calculus on a differentiable manifold and visualizing it. Likely start with a boring circle. That would be sweet...
Diffie-Hellman key exchange Updated 2025-07-16
Based on the fact that we don't have a P algorithm for the discrete logarithm of the cyclic group as of 2020, but we do have an efficient algorithm for modular exponentiation. But nor do we have proof that one does not exist! Living on the edge as usual for public-key cryptography.
Dimension Updated 2025-07-16
Diophantine equation Updated 2025-07-16
Sometimes systems of Diophantine equations are considered.
Problems generally involve finding integer solutions to the equations, notably determining if any solution exists, and if infinitely solutions exist.
The general problem is known to be undecidable: Hilbert's tenth problem.
The Pythagorean triples, and its generalization Fermat's last theorem, are the quintessential examples.
Dirac Lagrangian Updated 2025-07-16
where:
Remember that is a 4-vetor, gamma matrices are 4x4 matrices, so the whole thing comes down to a dot product of two 4-vectors, with a modified by matrix multiplication/derivatives, and the result is a scalar, as expected for a Lagrangian.
Like any other Lagrangian, you can then recover the Dirac equation, which is the corresponding equations of motion, by applying the Euler-Lagrange equation to the Lagrangian.
Discord email notifications Updated 2025-07-16
Discord is useless if you want to participate in more than one large group because of this. It is impossible to get email notification for selected threads you care about.
Divergence in Einstein notation Updated 2025-07-16
First we write a vector field as:
Note how we are denoting each component of as with a raised index.
Then, the divergence can be written in Einstein notation as:
It is common to just omit the variables of the function, so we tend to just say:
or equivalently when referring just to the operator:
Double-slit experiment Updated 2025-07-16
Amazingly confirms the wave particle duality of quantum mechanics.
The effect is even more remarkable when done with individual particles such individual photons or electrons.
Richard Feynman liked to stress how this experiment can illustrate the core ideas of quantum mechanics. Notably, he night have created the infinitely many slits thought experiment which illustrates the path integral formulation.

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