A geometric progression, also known as a geometric sequence, is a mathematical sequence of non-zero numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio. For example, the sequence 2, 6, 18, 54, ... is a geometric progression with common ratio 3. Similarly 10, 5, 2.5, 1.25, ... is a geometric sequence with common ratio 1/2.
Examples of a geometric sequence are powers rk of a fixed non-zero number r, such as 2k and 3k. The general form of a geometric sequence is
a, ar, ar2, ar3, ar4, \ldots
where r ≠ 0 is the common ratio and a ≠ 0 is a scale factor, equal to the sequence's start value.The sum of a geometric progression's terms is called a geometric series.
The n-th term of a geometric sequence with initial value a = a1 and common ratio r is given by
an=arn-1,
an=
| n-m | |
| a | |
| mr |
.
Such a geometric sequence also follows the recursive relation
an=ran-1
n\geq2.
Generally, to check whether a given sequence is geometric, one simply checks whether successive entries in the sequence all have the same ratio.
The common ratio of a geometric sequence may be negative, resulting in an alternating sequence, with numbers alternating between positive and negative. For instance
1, −3, 9, −27, 81, −243, ...is a geometric sequence with common ratio −3.
The behaviour of a geometric sequence depends on the value of the common ratio. If the common ratio is:
Geometric sequences (with common ratio not equal to −1, 1 or 0) show exponential growth or exponential decay, as opposed to the linear growth (or decline) of an arithmetic progression such as 4, 15, 26, 37, 48, … (with common difference 11). This result was taken by T.R. Malthus as the mathematical foundation of his Principle of Population.Note that the two kinds of progression are related: exponentiating each term of an arithmetic progression yields a geometric progression, while taking the logarithm of each term in a geometric progression with a positive common ratio yields an arithmetic progression.
An interesting result of the definition of the geometric progression is that any three consecutive terms a, b and c will satisfy the following equation:
b2=ac
where b is considered to be the geometric mean between a and c.
The product of a geometric progression is the product of all terms. It can be quickly computed by taking the geometric mean of the progression's first and last individual terms, and raising that mean to the power given by the number of terms. (This is very similar to the formula for the sum of terms of an arithmetic sequence: take the arithmetic mean of the first and last individual terms, and multiply by the number of terms.)
As the geometric mean of two numbers equals the square root of their product, the product of a geometric progression is:
| n | |
| \prod | |
| i=0 |
ari=(\sqrt{a ⋅ arn})n+1=(\sqrt{a2rn})n+1
(An interesting aspect of this formula is that, even though it involves taking the square root of a potentially-odd power of a potentially-negative, it cannot produce a complex result if neither nor has an imaginary part. It is possible, should be negative and be odd, for the square root to be taken of a negative intermediate result, causing a subsequent intermediate result to be an imaginary number. However, an imaginary intermediate formed in that way will soon afterwards be raised to the power of
stylen+1
Let represent the product. By definition, one calculates it by explicitly multiplying each individual term together. Written out in full,
P=a ⋅ ar ⋅ ar2 … arn-1 ⋅ arn
Carrying out the multiplications and gathering like terms,
P=an+1r1+2+3+
The exponent of is the sum of an arithmetic sequence. Substituting the formula for that calculation,
P=an+1
| ||||
| r |
which enables simplifying the expression to
P=
| ||||
| (ar |
)n+1=(a\sqrt{rn})n+1
Rewriting as
style\sqrt{a2}
P=(\sqrt{a2rn})n+1
which concludes the proof.
A clay tablet from the Early Dynastic Period in Mesopotamia (c. 2900 – c. 2350 BC), identified as MS 3047, contains a geometric progression with base 3 and multiplier 1/2. It has been suggested to be Sumerian, from the city of Shuruppak. It is the only known record of a geometric progression from before the time of old Babylonian mathematics beginning in 2000 BC.[1]
Books VIII and IX of Euclid's Elements analyzes geometric progressions (such as the powers of two, see the article for details) and give several of their properties.[2]