testtest 测试一下，看是否能正常显示

公式显示

什么是线性模型

对于模型：

$$Y_{i} = \beta_{0}+\beta_{1}\phi _{1}(X_)+\cdots +\beta _{p}\phi _{p}(X_)+\varepsilon _{i}\qquad i=1,\ldots ,n$$

where ${\displaystyle \phi _{1},\ldots ,\phi _{p}} \phi _{1},\ldots ,\phi _{p}$ may be nonlinear functions. In the above, the quantities εi are random variables representing errors in the relationship. The “linear” part of the designation relates to the appearance of the regression coefficients, βj in a linear way in the above relationship. Alternatively, one may say that the predicted values corresponding to the above model, namely

$${\hat {Y}}_{i}=\beta _{0}+\beta _{1}\phi _{1}(X_)+\cdots +\beta _{p}\phi _{p}(X_)\qquad (i=1,\ldots ,n),$$

are linear functions of the $\beta_j$.

https://en.wikipedia.org/wiki/Linear_model