The Fibonnaci sequence where each term is the sum of the preceding two terms, that is fn+2=fn+1+fn, has
the first two terms f0=0 and f1=1.
For which values of n is the Fibonnaci number fn even?
Which Fibonnaci numbers are divisible by 3?
Prove general results for the occurrence of even Fibonnaci numbers in the sequence and
for the occurrence of multiples of 3.