You can create a bijection between that set and the set of natural numbers, defined by the function f: 0 <-> sandwich 1 <-> 0 ... n+1 <-> n By definition,

You can create a bijection between that set and the set of natural numbers, defined by the function f: 0 <-> sandwich 1 <-> 0 ... n+1 <-> n By definition, if you can biject two sets, they have the same cardinality.