Geometric series
In mathematics, a geometric progression is a sequence of numbers such that the quotient of any two successive members of the sequence is a constant called the common ratio of the sequence.
Thus without loss of generality a geometric sequence can be written as where r is the common ratio and a is a scale factor. Thus the common ratio gives a family of geometric sequences whose starting value is determined by the scale factor.
For example, a sequence with a common ratio of 2 and a scale factor of 1 is
- 1, 2, 4, 8, 16, 32, 64, 128, 256, ....
The sum of the numbers in a geometric progression is called a geometric series. Thus the geometric series for the n terms of a geometric progression is
Multiplying by equals since all the other terms cancel in pairs.
Rearranging gives the general formula for the sum of a geometric series:
An interesting relationship for a geometric series is given by:
For example, (1 + 2 + 4 + 8 + 16 + 32 + 64 + 128 + 256 +...) can be written as (1 + 2 + 4)(1 + 8 + 64 +...)
A geometric series is a sum of terms in which two successive terms always have the same ratio, i.e., the sequence of terms is a geometric sequence. For example,
- 4 + 8 + 16 + 32 + 64 + 128 + 256 + ...
is a geometric series with common ratio 2. This is the same as 2 * 2x where x is increasing by one for each number. It is called a geometric series because it occurs when comparing the length, area, volume, etc. of a shape in different dimensions.
The sum of a geometric series whose first term is a power of the common ratio can be computed quickly with the formula
which is valid for all natural numbers m ≤ n and all numbers x≠ 1 (or more generally, for all elements x in a ring such that x − 1 is invertible). This formula can be verified by multiplying both sides with x - 1 and simplifying.
Using the formula, we can determine the above sum: (29 − 22)/(2 − 1) = 508. The formula is also extremely useful in calculating annuities: suppose you put $2,000 in the bank every year, and the money earns interest at an annual rate of 5%. How much money do you have after 6 years?
- 2,000 · 1.056 + 2,000 · 1.055 + 2,000 · 1.054 + 2,000 · 1.053 + 2,000 · 1.052 + 2,000 · 1.051
- = 2,000 · (1.057 − 1.05)/(1.05 − 1)
- = 14,284.02
An infinite geometric series is an infinite series whose successive terms have a common ratio. Such a series converges if and only if the absolute value of the common ratio is less than one; its value can then be computed with the formula
which is valid whenever |x| < 1; it is a consequence of the above formula for finite geometric series by taking the limit for n→∞.
This last formula is actually valid in every Banach algebra, as long as the norm of x is less than one, and also in the field of p-adic numbers if |x|p < 1.
Also useful is the formula
which can be seen as x times the derivative of the infinite geometric series. This formula only works for |x| < 1, as well.