===== Open ball =====
==== Set ====
| @#55CCEE: context     | @#55CCEE: $\langle d,X\rangle$ ... metric space |
| @#55CCEE: context     | @#55CCEE: $a\in X$ |

| @#FFBB00: definiendum | @#FFBB00: $B_a:\mathbb R_+^*\to \mathcal P(X)$ |
| @#FFBB00: definiendum | @#FFBB00: $B_a(r):= \{x\ |\ d(x,a)<r \}$ |

==== Discussion ====
=== Reference ===
Wikipedia: [[http://en.wikipedia.org/wiki/Ball_%28mathematics%29|Ball (Mathematics)]]
==== Parents ====
=== Context ===
[[Metric]]