Three letters, and one Greek letter that turns up all over math, physics and computing. This domain is for sale.
Repeat x ↦ x² + c mod n. Seen mod a hidden prime factor p, the walk has to repeat itself after about √p steps, and the path it traces looks like a ρ. A tortoise and a hare run the walk at different speeds, and gcd(x − y, n) catches the loop, which gives you p.
Pearson's r measures how well the points fit a straight line. Spearman's ρ measures whether y goes up when x goes up, at any rate. Drag the points around. Click empty space to add one, double-click a point to remove it.
Every field has its own ρ, which makes a name like this useful to almost anyone.
Finds a factor p in about √p steps using almost no memory.
Pearson's r computed on ranks. Robust to outliers and to curves.
Water at 4 °C is about 1000 kg/m³.
Copper is about 1.7 × 10⁻⁸ Ω·m at 20 °C.
x ← Ax + b converges from every starting point exactly when ρ(A) < 1.
Gauss's law: electric field lines begin and end where ρ is.
How much an option's price moves when the risk-free rate changes.
How much a series correlates with itself k steps back.