What is the term for a series of numbers with each having the same number added to it? example: 1, 4, 7, 10, 13?

The term for a series of numbers where you add the same number after each step is an Arithmetic Sequence. The general term of an Arithmetic Sequence is in the form a(n) = a(0) + d*n. If the series is 1, 4, 7, 10, ... then your a(0) is 1, and your d is 3. You must always specify the first term, or else you won't know where to start.