O () and O () represent the same order infinitesimal and higher order infinitesimal respectively.

A and b are infinitesimal. If the limit of b/a is equal to 0, it is said that b is infinitely smaller than a, and it is recorded as b=o(a). If the limit of b/a is equal to c(c≠0), then b and a are called infinitesimal of the same order, and it is recorded as b=O(a).