What's the mathematical formula for the sequence 3 3 6 6 12 12 24 24 etc?

Question

The closest I could find is $y=2^{\operatorname{ceil}\left(0.5x\right)}\frac{3}{2}x$ but its not quite right.

I possible, I'd like to not use sums (∑).

Answer 1

$3 \cdot2^{ceil(0.5*x)-1}$, for $x=1,2,\ldots$

Answer 2

You can use, for $n\geq 1$, $$a_n = 3\cdot 2^{\lfloor\frac{n-1}{2}\rfloor}$$ where $\lfloor x\rfloor$ is the floor of $x$.

Answer 3

For $n\ge 1,$

$$a_{2n}=a_{2n-1}=3.2^{n-1}$$