254. $f(x)=\ln ^{3}\left(x^{2}-1\right)$.

254. $f(x)=\ln ^{3}\left(x^{2}-1\right)$. 

The translation is the same as the original text because it is a mathematical expression which is universal and does not change when translated into English. However, if you meant to have the problem number and the function description translated, it would be:

254. $f(x)=\ln ^{3}\left(x^{2}-1\right)$.

254. $f(x)=\ln ^{3}\left(x^{2}-1\right)$. 

In this case, the text is already in a form that is commonly used in English mathematical literature.