A set is a collection of elements (\(\{\xi_1, \xi_2, ..., \xi_n\}\))
Elements can be anything… apples, real numbers, functions, sets!
B is a subset of A, denoted \(B \subseteq A\) if every member of B is also a member of A.
A <- c(1,2,3,4,5,6) B <- c(2,4,6) all(B %in% A)
## [1] TRUE
all(A %in% B)
## [1] FALSE