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: formula_6. (Or simply omega formula_7.) The infinite series ending "... third, second, first" is formula_8.