Recursion and Closed Formulas

Hello!

I've be scanning different help forums and I have a come across two different questions involving finding closed formulas. I have attempted to answer them with no luck and now I am curious of the answer.

The questions are --> Find a closed formula for the sequence:

a) 0,1,3,0,1,3,0,1,3,0,1,3,0,.... where a0 = 0, a1 = 1, a2 = 2, a3 = 0,...

b) an = an-1 + 3an-2, where a0 = 0 and a1 = 1

Perhaps one of you could shed some light on these questions. I appreciate your time. Thank you in advance!