他们也有很多朋友，所以他们每个星期都参加一二次聚会。

They also have a lot of friends, so they go to parties once or twice a week.

一二次空气污染物是一种有害化学形成了在大气中经过化学反应之间的空气成分。

A secondary air pollutant is a harmful chemical formed in the atmosphere through a chemical reaction among air components.

本文将非线性观测值函数泰勒级数展开，并取至二次项，得到线性一二次项形式。

This paper expands the non-linear observation function in Taylor's series, takes to the quadratic term and obtains the form of the linear-quadratic term.