Find the missing number in the following sequence:
$$13,1,17,\underline{},21$$

The sequence consists of two alternating sub-series:
1. First alternating series ($1^{\text{st}},3^{\text{rd}},5^{\text{th}}$ terms): $13,17,21$ (increases by $+4$ each step)

$$13+4=17$$

$$17+4=21$$

2. Second alternating series ($2^{\text{nd}},4^{\text{th}}$ terms): $1,\underline{}$
Applying the same constant rule of $+4$:

$$1+4=5$$

Therefore, the missing number is $5$.