Infinity
A different type of infinity are the ordinal numbers, beginning "first, second, third, ...". The order "first, second, third, ..." and so on to infinity is different from the order ending "..., third, second, first". The difference is important for mathematical induction. The simple first, second, third, ...  has the mathematical name: the Greek letter omega with subscript zero: . (Or simply omega .) The infinite series ending "... third, second, first" is .