2.5 Infinite Sets Definition We call a set XXX countably infinite if there exists an 1-1 function f:N→xf: N\to xf:N→x, where NNN is positive integers. X={2,4,6,8⋯},f(n)=2nX=\{2, 4, 6, 8 \cdots\}, f(n) = 2nX={2,4,6,8⋯},f(n)=2n,