What's the mathematical formula for the sequence 3 6 12 18 30 42 66 90 ...?

Question

What's the mathematical formula for the sequence 3 6 12 18 30 42 66 90 ... ? I can provide more numbers if necessary but the pattern is that the first number is exponentially doubling every two steps.

Answer 1

The general formula is $$a_n=(\frac{15}{2}+\frac{3}{2}\cdot (-1)^{n+1})\cdot 2^{\lceil\frac{n-1}{2}\rceil}-6$$

Answer 2

Thanks for all the help. Using the links provided in the comments I was able to find the following formula $y\ =\ 3\ \cdot2^{\frac{x-3}{2}}\left(\left(2\cdot\sqrt{2}-3\right)\cdot\left(-1\right)^{x}+3+2\sqrt{2}\right)-6$