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Computer Science @CompSciFact

Daily tweets about computer science and related stuff @JohnDCook.

Joined November 2010
  • Tweets 13,123
  • Following 17
  • Followers 251,033
662 Photos and videos
RSA munitions T-shirt johndcook.com/blog/2026/06/1…
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Does DNA have the equivalent of IF-statements, WHILE loops, or function calls? biology.stackexchange.com/qu…

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Every cellular automaton is a local automaton. -- Curtis Hedlund Lyndon theorem
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'Security is both a feeling and a reality. And they’re not the same.' -- Bruce Schneier
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The linear algebra of bit twiddling johndcook.com/blog/2026/05/1…
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Four views of multisets johndcook.com/blog/2022/10/2…

Four views of multisets

Four operations on multisets -- union, intersection, inclusion, sum -- from four perspectives -- words, Python code, prime factors, and functions.

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Solving a chess puzzle with Claude and Prolog johndcook.com/blog/2026/06/1…
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Formally proving a calculus calculation with Claude and Lean johndcook.com/blog/2026/06/1…

Formally proving a calculation with Claude and LEAN

Using Claude Pro to generate a LEAN proof of a calculus calculation, namely the sine series coefficients for the solution to Kepler's equation.

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'Beware of bugs in the above code; I have only proved it correct, not tried it.' -- Donald Knuth
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'Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function.' -- John Carmack
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Gray code johndcook.com/blog/2020/09/0…

Gray codes: list binary numbers changing one bit at a time

Gray codes are a way of encoding integers so that the encodings of consecutive integers differ in exactly one bit.

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A graph is k-connected if it remains connected when any k-1 vertices are removed.
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'The psychological profiling [of a programmer] is mostly the ability to shift levels of abstraction, from low level to high level. To see something in the small and to see something in the large.' -- Donald Knuth
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State machines blog.markshead.com/869/state…
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The halting problem: You cannot write a program that can tell whether an arbitrary program will complete or run forever.
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'Methods and object can no longer be separated.' -- Werner Heisenberg
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Converting between quaternions and rotation matrices johndcook.com/blog/2025/05/0…

Converting between quaternions and rotation matrices

Equations and Python code for going back and forth between quaternion and matrix representations of rotations.

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“Cryptography is a mixture of mathematics and muddle, and without the muddle the mathematics can be used against you.” — Ian Cassels
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'Writing code is often a constant struggle against distraction.' -- Joy of Clojure
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Classic cryptography: The Playfair cipher johndcook.com/blog/2023/02/2…

Playfair cipher

The Playfair cipher, invented by Wheatstone, was the first encryption method to encrypt text two letters at a time. Symmetry makes it easy to use and to break.

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