Geometric progression

The nth term of a geometric sequence with initial value a = a₁ and common ratio r is given by

${\displaystyle a_{n}=a\,r^{n-1},}$ 

and in general

${\displaystyle a_{n}=a_{m}\,r^{n-m}.}$ 

Geometric sequences satisfy the linear recurrence relation

${\displaystyle a_{n}=r\,a_{n-1}}$  for every integer ${\displaystyle n>1.}$ 

This is a first order, homogeneous linear recurrence with constant coefficients.

Geometric sequences also satisfy the nonlinear recurrence relation

${\displaystyle a_{n}=a_{n-1}^{2}/a_{n-2}}$  for every integer ${\displaystyle n>2.}$ 

This is a second order nonlinear recurrence with constant coefficients.

When the common ratio of a geometric sequence is positive, the sequence's terms will all share the sign of the first term. When the common ratio of a geometric sequence is negative, the sequence's terms alternate between positive and negative; this is called an alternating sequence. For instance the sequence 1, −3, 9, −27, 81, −243, ... is an alternating geometric sequence with an initial value of 1 and a common ratio of −3. When the initial term and common ratio are complex numbers, the terms' complex arguments follow an arithmetic progression.

If the absolute value of the common ratio is smaller than 1, the terms will decrease in magnitude and approach zero via an exponential decay. If the absolute value of the common ratio is greater than 1, the terms will increase in magnitude and approach infinity via an exponential growth. If the absolute value of the common ratio equals 1, the terms will stay the same size indefinitely, though their signs or complex arguments may change.

Geometric progressions show exponential growth or exponential decline, as opposed to arithmetic progressions showing linear growth or linear decline. This comparison was taken by T.R. Malthus as the mathematical foundation of his An Essay on the Principle of Population. The two kinds of progression are related through the exponential function and the logarithm: exponentiating each term of an arithmetic progression yields a geometric progression, while taking the logarithm of each term in a geometric progression yields an arithmetic progression.

The infinite product of a geometric progression is the product of all of its terms. The partial product of a geometric progression up to the term with power ${\displaystyle n}$  is

$${\displaystyle \prod _{k=0}^{n}ar^{(k)}=a^{n+1}r^{n(n+1)/2}.}$$ 

When ${\displaystyle a}$  and ${\displaystyle r}$  are positive real numbers, this is equivalent to taking the geometric mean of the partial progression's first and last individual terms and then raising that mean to the power given by the number of terms ${\displaystyle n+1.}$ 

$${\displaystyle \prod _{k=0}^{n}ar^{k}=a^{n+1}r^{n(n+1)/2}=({\sqrt {a^{2}r^{n}}})^{n+1}{\text{ for }}a\geq 0,r\geq 0.}$$ 

This corresponds to a similar property of sums of terms of a finite arithmetic sequence: the sum of an arithmetic sequence is the number of terms times the arithmetic mean of the first and last individual terms. This correspondence follows the usual pattern that any arithmetic sequence is a sequence of logarithms of terms of a geometric sequence and any geometric sequence is a sequence of exponentiations of terms of an arithmetic sequence. Sums of logarithms correspond to products of exponentiated values.

Let ${\displaystyle P_{n}}$  represent the product up to power ${\displaystyle n}$ . Written out in full,

${\displaystyle P_{n}=a\cdot ar\cdot ar^{2}\cdots ar^{n-1}\cdot ar^{n}}$ .

Carrying out the multiplications and gathering like terms,

${\displaystyle P_{n}=a^{n+1}r^{1+2+3+\cdots +(n-1)+n}}$ .

The exponent of r is the sum of an arithmetic sequence. Substituting the formula for that sum,

${\displaystyle P_{n}=a^{n+1}r^{\frac {n(n+1)}{2}}}$ ,

which concludes the proof.

One can rearrange this expression to

${\displaystyle P_{n}=(ar^{\frac {n}{2}})^{n+1}.}$ 

Rewriting a as ${\displaystyle \textstyle {\sqrt {a^{2}}}}$  and r as ${\displaystyle \textstyle {\sqrt {r^{2}}}}$  though this is not valid for ${\displaystyle a<0}$  or ${\displaystyle r<0,}$ 

${\displaystyle P_{n}=({\sqrt {a^{2}r^{n}}})^{n+1}{\text{ for }}a\geq 0,r\geq 0}$ 

which is the formula in terms of the geometric mean.

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 analyze geometric progressions (such as the powers of two, see the article for details) and give several of their properties.^[2]

• Arithmetic progression – Sequence of equally spaced numbers
• Arithmetico-geometric sequence – Mathematical sequence satisfying a specific pattern
• Linear difference equation – Relation in AlgebraPages displaying short descriptions of redirect targets
• Exponential function – Mathematical function, denoted exp(x) or e^x
• Harmonic progression – Progression formed by taking the reciprocals of an arithmetic progression
• Harmonic series – Divergent sum of all positive unit fractions
• Infinite series – Infinite sum
• Preferred number – Standard guidelines for choosing exact product dimensions within a given set of constraints
• Thomas Robert Malthus – British political economist (1766–1834)
• Geometric distribution – Probability distribution

• Hall & Knight, Higher Algebra, p. 39, ISBN 81-8116-000-2

• "Geometric progression", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
• Derivation of formulas for sum of finite and infinite geometric progression at Mathalino.com
• Geometric Progression Calculator Archived 2008-12-27 at the Wayback Machine
• Nice Proof of a Geometric Progression Sum at sputsoft.com
• Weisstein, Eric W. "Geometric Series". MathWorld.