Geometric progression
In mathematics, a geometric progression, also known as a geometric sequence, is a 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 "r" of a fixed non-zero number "r", such as 2 and 3. The general form of a geometric sequence is  where "r" ≠ 0 is the common ratio and "a" ≠ 0 is a scale factor, equal to the sequence's start value. The distinction between a progression and a series is that a progression is a sequence, whereas a series is a sum. Elementary properties. The "n"-th term of a geometric sequence with initial value "a" = "a" and common ratio "r" is given by Such a geometric sequence also follows the recursive relation 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 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: where "b" is considered to be the "geometric mean" between "a" and "c". Geometric series. A geometric series is the sum of the numbers in a geometric progression. For  example: Letting "a" be the first term (here 2), n be the number of terms (here 4), and "r" be the constant that each term is multiplied by to get the next term (here 5), the sum is given by: In the example above, this gives: The formula works for any real numbers "a" and "r" (except "r" = 1, which results in a division by zero). For example: Since the derivation (below) does not depend on "a" and "r" being real, it holds for complex numbers as well. Derivation. To derive this formula, first write a general geometric series  as: We can find a simpler formula for this sum by multiplying both sides of the above equation by 1 − "r", and we'll see that since all the other terms cancel. If "r" ≠ 1, we can rearrange the above to get the convenient formula for a geometric series that computes the sum of n terms: Related formulas. If one were to begin the sum not from k=1, but from a different value, say "m", then provided formula_14. If formula_15 then the sum is of just the constant formula_16 and so equals formula_17. Differentiating this formula with respect  to "r" allows us to arrive at formulae for sums of the form For example: For a geometric series containing only even powers of "r" multiply by  1 − "r"  : Then Equivalently, take  "r"  as the common ratio and use the standard formulation. For a series with only odd powers of "r" and An exact formula for the generalized sum formula_24 when formula_25 is expanded by the Stirling numbers of the second kind as Infinite geometric series. 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 ( < 1). Its value can then be computed from the finite sum formula Since: Then: For a series containing only even powers of formula_30, and for odd powers only, In cases where the sum does not start at "k" = 0, The formulae given above are valid only for  < 1. The latter formula is valid in every Banach algebra, as long as the norm of "r" is less than one, and also in the field of "p"-adic numbers if  < 1. As in the case for a finite  sum, we can differentiate to calculate formulae for related sums. For example, This formula only works for  < 1 as well. From this, it follows that, for  < 1, Also, the infinite series 1/2 + 1/4 + 1/8 + 1/16 + ⋯ is an elementary example of a series that converges absolutely. It is a geometric series whose first term is 1/2 and whose common ratio is 1/2, so its sum is The inverse of the above series is 1/2 − 1/4 + 1/8 − 1/16 + ⋯ is a simple example of an alternating series that converges absolutely. It  is a geometric series whose first term is 1/2 and whose common ratio is −1/2, so its sum is Complex numbers. The summation formula for geometric series remains valid even when the common ratio is a complex number. In this case the condition that the absolute value of "r" be less than 1 becomes that the modulus of "r" be less than 1. It is possible to calculate the sums of some non-obvious geometric series. For example, consider the proposition The proof of this comes from the fact that which is a consequence of Euler's formula. Substituting this  into the original series gives This is the difference of two geometric series, and so it is a straightforward application of the formula for infinite geometric series that completes the proof. Product. 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: Proof. Let represent the product. By definition, one calculates it by explicitly multiplying each individual term together. Written out in full, Carrying out the multiplications and gathering like terms, The exponent of is the sum of an arithmetic sequence. Substituting the formula for that calculation, which enables simplifying the expression to Rewriting as formula_47, which concludes the proof. History. A clay tablet from the Early Dynastic Period in Mesopotamia , 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 Babylonian mathematics. 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.