Open ball

Set

context $\langle d,X\rangle$ … metric space
context $a\in X$
definiendum $B_a:\mathbb R_+^*\to \mathcal P(X)$
definiendum $B_a(r):= \{x\ |\ d(x,a)<r \}$

Discussion

Reference

Wikipedia: Ball (Mathematics)

Parents

Context

Metric