Modular arithmetic

2 articles

Combinatorics & Discrete Math LycéeExplication9 min

Say Six: The Only Opening That Wins the Race to Fifty

Counting to fifty in steps of one to ten, the first player wins, and exactly one of the ten legal openings does it. The stations are 6, 17, 28, 39 and 50, spaced eleven apart because eleven is one more than the largest legal step. The article carries the residue argument that proves the opening is unique, and the target 55 where the advantage flips.

Number Theory & Arithmetic LycéeExplication9 min

2519, and the Shift That Turns Nine Congruences Into One

Every one of the nine conditions leaves a remainder one short of its divisor, so x plus one is divisible by all of 2 through 10 and the answer is 2519. Minimality comes free, and the whole solution set is 2520k minus 1. The article carries the coprimality caveat, the near miss 209 that satisfies six of the nine, and a variant where no shift exists.