
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
If a sequence $\{a_n\}$ satisfies $a_{n-1}+a_{n+1} \geqslant 2a_n$ $(n \geqslant 2)$, then the sequence $\{a_n\}$ is called a concave sequence. Given that an arithmetic sequence $\{b_n\}$ has a common difference $d$, $b_1=4$, and the sequence $\left\{ \frac{b_n}{n} \right\}$ is a concave sequence, then the range of values for $d$ is _______.

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