i need to write a recursive formula for the nth term of the sequence:

2, 5, 7, 12, 19, 31,...

i can see the pattern they are increasing by but i'm horrible at figuring out how to put it into a formula... i just know how to set it up:

$\displaystyle a_1$=2 $\displaystyle a_n$=...?

any advice on how to start?