50 hard puzzles that test logic, mathematics and intuition
Scales, coins, hourglasses, strategies and paradoxes: fifty challenges whose results and explanations stay hidden until you choose to reveal them.
ReadMathematics
Articles by topic
Clear, organized learning paths that build strong foundations in mathematics and computer science.
Logic
Algebra
Baker’s transformation: chaos that mixes images
Stretch, compress and reassemble: use images and points to see how a deterministic rule produces surprising mixing.
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Number theory
Cryptography: four methods, from Caesar to Alice and Bob
Substitution, transposition, XOR and RSA: full encryption and decryption examples with an interactive laboratory.
ReadThe Chinese remainder theorem: from Sunzi to 30 problems
Sunzi’s block method, modern congruences, and 30 interactive challenges involving coins, soldiers, calendars, and trick questions.
ReadFrom 3 and 5 to primes in arithmetic progression
Fajtlowicz’s conjecture, two ways to generate primes from 3 and 5, and a laboratory that also accepts a new starting pair.
ReadModular arithmetic: the remainders laboratory
From clocks to enormous powers: an intuitive guide to congruences with 25 problems and an interactive laboratory.
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Algorithms
From 2010 to 2026: the inheritance problem generalized
From Fibonacci’s puzzle to a formula for every child: proof of the general case, four checked examples, and the roles of a and d.
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Probability
Combinatorics: from first counts to complex problems
A step-by-step path with examples, justified formulas, a laboratory and an explained-answer quiz.
ReadFrom data to distributions and confidence intervals
A sample, a histogram, several possible models and an interval for the mean: learn what the data can tell us, and what they cannot.
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