I try to solve a recursive formula and find the explicit representation of a

$ a_0 = 1$

$ a_{n+1} = 3 \sum\limits_{i=0}^{n} a_i$

`RSolve[{a[n] == 3 Sum[a[i], {i, 0, n }], a[0] == 1}, a[n], n] `

and the output is

`RSolve: 2\ a[-1+n] cannot be used as a function. `