Finding expressions for sequences

example: 2,4, 8, 16, 32 ...is 2^n

where n= rank

example: 3, 9, 27, 81 ...is 3^n

soo what is the expression for 4, 8, 32

you can express it in form of 2 to a certain power

such that it is: 2^2 , 2^3, 2^5

i tried 2^2n-1, it works for the 2nd and 3rd value but not the 1st ???

OR it can be noticed that the powers are prime numbers

so is the expression 2^p(n)

ANY ideas????

thnxx in advance