Find the rule in this number series: 4,7,12,21,36,63,108,189,336,615?

Answer 1

Well it looks like two sequences combined to me, except 336 doesn't fit... are you sure it's not 324?

One sequence is 4, 12, 36, 108, ... It starts with 4 and increases by a factor of 3 each time.... except for 336 (it should be 324)?

The other sequence is 7, 21, 63, 189, ...I can't tell what your last number was but it should be 567 if the trend continues....? It looks like it starts with 61, which again doesn't fit...?

Anyway my best guess would be:

an = 4(3)^(floor(n/2)-1), when n is odd, and
an = 7(3)^(n/2-2), when n is even