1 Regarding the mark o, when x →a, there is a mark α(x) = o [β(x)] between two infinitesimal quantities α(x) and β(x), that is, when x →a, the infinitesimal quantity α (x) is infinitely small relative to β (x), that is, Especially when x →a and f(x) →0, it is marked as f(x)=o( 1). Often used in x →a and f(x) →A, it is marked as f(x)=A+o( 1).

2 About the notation o, that is, when x →a, f(x)/g(x) remains bounded, so it is recorded as f(x) = O[g(x)]. In particular, the symbol f(x) = O( 1) represents the bounded quantity when f(x) is x →a a.