Is the sequence
(with a_{1}=0 and a_{n}=a_{n-1}-n)

a_{1}=0

a_{2}=a_{1}-2=0-2=-2

a_{3}=a_{2}-3=-2-3=-5

a_{4}=a_{3}-4=-5-4=-9

a_{5}=a_{4}-5=-9-5=-14

First differences=-2, -3, -4, -5…

Second differences=-1, -1, -1…

For this sequence, since the second differences are all the same, the sequence has a perfect quadratic model. (if the first differences were to be all the same, the sequence has a linear model)