Lucky Number Slevin | Reynolds number | Mach number | serial number | Random number generation | International Standard Book Number | prime number | Personal identification number | number | Two in a Million/You're My Number One | Social Security number | Rikki Don't Lose That Number | Prime number | Premium-rate telephone number | Jackass Number Two | Opus number | Number Girl | List of WTA number 1 ranked players | Lah number | Erdős number | Emergency telephone number | Betti number | You're My Number One | What's Your Number? | The Number 23 | The Murderer Lives at Number 21 | real number | opus number | natural number | National identification number |
Chromatic number of a graph, sometimes (Chi) is used, which is also used for the Euler characteristic
Hadwiger–Nelson problem on the chromatic number of unit distance graphs in the Euclidean plane