
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given the function $f(x)=\ln x-ax$, where $a > 0$.

(1) When $a=1$, find the monotonic interval of $f(x)$;

(2) If the maximum value of function $f(x)$ is $-4$ when $1\leqslant x\leqslant e$, find the expression of function $f(x)$.

Present the answer in LaTex format: \boxed{Your answer}
user