Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

necklace

A circular arrangement where both rotation and reflection are considered the same, dividing the circular count by an extra 2.

A necklace of $n$ distinct beads, rotations and flips identified, has $(n-1)!/2$ distinct arrangements for $n \geq 3$ .