I have the following sequence of numbers:

1 3 6 10 15...

It isn't an AP because the difference isn't constant.

It starts in 2, then 3, 4, 5 and so on. Changing 1 each time.

1) Is there a way to know if a number is in this sequence? For instance, is 55 in this sequence? (it is...)

2) What is the difference used to reach some number in the sequence? For instance, the number 55, what is the difference that was used to find it (it is 10, the previous number is 45).

Thanks.