Divergence Updated 2025-07-16
Takes a vector field as input and produces a scalar field.
Mnemonic: it gives out the amount of fluid that is going in or out of a given volume per unit of time.
Therefore, if you take a cubic volume:
  • the input has to be the 6 flows across each face, therefore 3 derivatives
  • the output is the variation of the quantity of fluid, and therefore a scalar
Stack Exchange Updated 2025-07-16
Waste of time sub-sites that should instead be merged into Stack Overflow as different tags.
Nowhere is this waste more visible than at: cs50.stackexchange.com/. A website just for some specific course that is completely covered by other sites of the network? What a humongous waste!!!
GNU Privacy Guard Updated 2025-07-16
Generate public private key, test encrypt and test decrypt:
# Create your pubkey.
gpg --gen-key
gpg --armor --output pubkey.gpg --export <myemail>

# Encrypt using someone's pubkey.
gpg --import pubkey2.gpg
echo 'hello world' > hello.txt
gpg --output hello.txt.gpg --encrypt --recipient <other-email> hello.txt

# Double check it is not plaintext in the encrypted message.
grep hello hello.txt.gpg

# Decrypt.
gpg --output hello.decrypt.txt --decrypt --recipient <myemail> hello.txt.gpg
diff -u hello.decrypt.txt hello.txt
Non Abelian anyon Updated 2025-07-16
Exotic and hard to find experimentally.
Rubik's Cube solution Updated 2025-07-16
Video 1.
The algorithmic trick that solves Rubik's Cubes and breaks ciphers by polylog (2022)
Source. Talks about the Meet-in-the-middle algorithm.
Carbohydrate Updated 2025-07-16
Carbonate Updated 2025-07-16
Corollary Updated 2025-07-16
An easy to prove theorem that follows from a harder to prove theorem.

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