
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
Let $m$ be a non-negative integer, and the units digit of $m$ is denoted as $G(m)$, such as $G(2014)=4$, $G(17)=7$, $G(0)=0$. This function is called the units digit function. The following statements about the units digit function are given: 
- $①G(a-b)=G(a)-G(b)$; 
- $②\forall a, b, c \in \mathbb{N}^*, \text{ if } a-b=10c, \text{ then } G(a)=G(b)$; 
- $③G(a\cdot b\cdot c)=G(G(a)\cdot G(b)\cdot G(c))$; 
- $④G(3^{2015})=9.$ 

The correct statement number(s) is/are __________.

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