The sequence is, in fact, quadratic. It is described by the equation
.. a[n] = (n*(n -1))/2 +2
First differences are increasing, so the sequence will not be arithmetic. An arithmetic sequence has constant first differences.
Second differences are constant, so the sequence will not be geometric. A geometric sequence will have first-, second-, third-differences, and those to any level, that have the same constant ratio as the terms of the original sequence.
