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In mathematics, a geometric progression, also known as a geometric sequence, is a sequence of 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. The sum of the terms of a geometric progression, or of an initial segment of a geometric progression, is known as a geometric series.
Thus, the general form of a geometric sequence is
and that of a geometric series is
where r ≠ 0 is the common ratio and a is a scale factor, equal to the sequence's start value.
The n-th term of a geometric sequence with initial value 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 series may be negative, resulting in an alternating sequence, with numbers switching from positive to negative and back. 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 a geometric progression is that for any value of the common ratio, 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.
|− (||10||+||50||+||250||+||1250||=||5 × 312 )|
|2||−||1250||=||(1 − 5) × 312|
A geometric series is the sum of the numbers in a geometric progression. For example:
Letting a be the first term (here 2), m 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:
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+1 terms:
If one were to begin the sum not from k=0, but from a different value, say m, then
Differentiating this formula with respect to r allows us to arrive at formulae for sums of the form
For a geometric series containing only even powers of r multiply by 1 − r2 :
Equivalently, take r2 as the common ratio and use the standard formulation.
For a series with only odd powers of r
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 ( | r | < 1 ). Its value can then be computed from the finite sum formulae
For a series containing only even powers of ,
and for odd powers only,
In cases where the sum does not start at k = 0,
The formulae given above are valid only for | r | < 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 | r |p < 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 | r | < 1 as well. From this, it follows that, for | r | < 1,
It is a geometric series whose first term is 1/2 and whose common ratio is 1/2, so its sum is
It is a geometric series whose first term is 1/2 and whose common ratio is −1/2, so its sum is
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.
The product of a geometric progression is the product of all terms. If all terms are positive, then it can be quickly computed by taking the geometric mean of the progression's first and last term, 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 term and multiply with the number of terms.)
Let the product be represented by P:
Now, carrying out the multiplications, we conclude that
Applying the sum of arithmetic series, the expression will yield
We raise both sides to the second power:
which concludes the proof.
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A geometric progression is given this name because each term is the geometric mean of the two adjacent terms.
The geometric progression 1, 2, 4, 8, 16, 32, … (or, in the binary numeral system, 1, 10, 100, 1000, 10000, 100000, … ) is important in number theory. Book IX, Proposition 36 of Elements proves that if the sum of the first n terms of this progression is a prime number, then this sum times the nth term is a perfect number. For example, the sum of the first 5 terms of the series 1 + 2 + 4 + 8 + 16 = 31, which is a prime number. The sum 31 multiplied by 16 (the 5th term in the series) equals 496, which is a perfect number.
Book IX, Proposition 35, proves that in a geometric series if the first term is subtracted from the second and last term in the sequence then as the excess of the second is to the first, so will the excess of the last be to all of those before it. (This is a restatement of our formula for geometric series from above.) Applying this to the geometric progression 31, 62, 124, 248, 496 (which results from 1, 2, 4, 8, 16 by multiplying all terms by 31), we see that 62 minus 31 is to 31 as 496 minus 31 is to the sum of 31, 62, 124, 248. Therefore the numbers 1, 2, 4, 8, 16, 31, 62, 124 and 248 add up to 496 and further these are all the numbers which divide 496. For suppose that p divides 496 and it is not amongst these numbers. Assume pq equals 16 × 31, or 31 is to q as p is to 16. Now p cannot divide 16 or it would be amongst the numbers 1, 2, 4, 8 or 16. Therefore 31 cannot divide q. And since 31 does not divide q and q measures 496, the fundamental theorem of arithmetic implies that q must divide 16 and be amongst the numbers 1, 2, 4, 8 or 16. Let q be 4, then p must be 124, which is impossible since by hypothesis p is not amongst the numbers 1, 2, 4, 8, 16, 31, 62, 124 or 248.