There is no question asked, and the statement is incomplete, but this describes Series and Sequence. They are usually arithmetic or geometric progressions. A set of numbers are written to follow a particular pattern, when the pattern is clear, one can find any number of the sequence or series.
For example, consider the sequence below (Arithmetic Progression)
2, 4, 6, 8, ...
This sequence has a particular pattern.
The first term of the sequence is
a = 2
The common difference d, which is the difference between any two consecutive terms is
d = 2
The nth term, which is the formula that defines every single term of the sequence is given as
Tn = a + (n - 1)d
Check
T1 = a = 2
T2 = 2 + 2 = 4
T3 = 2 + 2(2) = 6
T4 = 2 + 2(3) = 8
T20 = 2 + 2(19) = 40
And so on.
Geometric progression
2, 4, 8, 16, 32, etc
First term, a = 2
Common ratio r, is the division of a particular number by the number preceding it.
r = 4/2 = 8/4 = 16/8 = 32/16 = 2
The nth term is given as
Sn = ar^(n-1)
S1 = a = 2
S2 = 2×2 = 4
S3 = 2×2^(2) = 8
S4 = 2×2^(3) = 16
S5 = 2×2^(4) = 32
S20 = 2×2^(19) = 1048576
And so on.