I'm to find an explicit formula for this sequence:

a_1 = 0

a_2 = 4

a_3 = 12

a_4 = 52

a_5 = 204

a_6 = 820

...

(ie "add 1 and muliplty by 4; subtract 1 and multiply by 4; add 1 and muliplty by 4; subtract 1 and multiply by 4 etc etc")

But can an explicit formula for a_n in terms of n (I don't want a "recurrent" formula based on a_(n-1), obviously) be found?