对于一个函数 ，如果对于任意一个正多项式，存在一个，使得对于所有的

那幺我们说这个函数是可忽略的（negligible）。通常我们把“存在一个，使得对于所有的”简化为“对于所有足够大的”。

In mathematics, a negligible function is a function such that for every positive integer c there exists an integer N_c such that for all x > N_c,

Equivalently, we may also use the following definition. A function is negligible, if for every positive polynomial poly(·) there exists an integer N_poly > 0 such that for all x > N_poly