Generalizations of Fibonacci numbers
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- F(0) = 0
- F(1) = 1
- F(n) = F(n-1) + F(n-2), for integer n > 1.
That is, after two starting values, each number is the sum of the two preceding numbers.
The Fibonacci sequence has been studied extensively and generalized in many ways, for example, by starting with other numbers than 0 and 1, by adding more than two numbers to generate the next number, or by adding objects other than numbers.
Extension to negative integers
Using Fn-2 = Fn - Fn-1, one can extend the Fibonacci numbers to negative integers. So we get: ... -8, 5, -3, 2, -1, 1, 0, 1, 1, 2, 3, 5, 8, ... and F-n = -(-1)nFn.
See also Negafibonacci.
Extension to all real or complex numbers
There are a number of possible generalizations of the Fibonacci numbers which include the real numbers (and sometimes the complex numbers) in their domain. These each involve the golden ratio φ, and are based on Binet's formula
The analytic function
has the property that Fe(n) = Fn for even integers n.  Similarly, the analytic function:
satisfies Fo(n) = Fn for odd integers n.
Finally, putting these together, the analytic function
satisfies Fib(n)=Fn for all integers n. 
This formula can be used to compute the generalized Fibonacci function of a complex variable. For example,
The term Fibonacci sequence is also applied more generally to any function g from the integers to a field for which g(n+2) = g(n) + g(n+1). These functions are precisely those of the form g(n) = F(n)g(1) + F(n-1)g(0), so the Fibonacci sequences form a vector space with the functions F(n) and F(n-1) as a basis.
Similar integer sequences
Fibonacci integer sequences
The 2-dimensional Z-module of Fibonacci integer sequences consists of all integer sequences satisfying g(n+2) = g(n) + g(n+1). Expressed in terms of two initial values we have:
- g(n) = F(n)g(1) + F(n-1)g(0) =
where is the golden ratio.
Written in the form
a = 0 if and only if b = 0.
Thus the ratio between two consecutive elements converges to the golden ratio, except in the case of the sequence which is constantly zero.
Written in this form the simplest non-trivial example (a = b = 1) is the sequence of Lucas numbers:
We have L(1) = 1 and L(2) = 3. The properties include:
See also Fibonacci integer sequences modulo n.
A generalization of the Fibonacci sequence are the Lucas sequences of the kind defined as follows:
- U(0) = 0
- U(1) = 1
- U(n + 2) = PU(n + 1) − QU(n)
where the normal Fibonacci sequence is the special case of P = 1 and Q = −1. Another kind of Lucas sequence begins with V(0) = 2, V(1) = P. Such sequences have applications in number theory and primality proving.
Fibonacci numbers of higher order
A Fibonacci sequence of order n is an integer sequence in which each sequence element is the sum of the previous n elements (with the exception of the first n elements in the sequence). The usual Fibonacci numbers are a Fibonacci sequence of order 2. The cases n=3 and n=4 have been thoroughly investigated. The number of compositions of nonnegative integers into parts that are at most n is a Fibonacci sequence of order n.
The tribonacci numbers are like the Fibonacci numbers, but instead of starting with two predetermined terms, the sequence starts with three predetermined terms and each term afterwards is the sum of the preceding three terms. The first few tribonacci numbers are:
- 0, 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, 274, 504, 927, 1705, 3136, 5768, 10609, 19513, 35890, 66012, … (sequence A000073 in OEIS)
The tribonacci constant is the ratio toward which adjacent tribonacci numbers tend. It is a root of the polynomial x3 − x2 − x − 1, approximately 1.83929 (sequence A058265 in OEIS), and also satisfies the equation x + x−3 = 2. It is important in the study of the snub cube.
The tribonacci numbers are also given by
where the outer brackets denote the nearest integer function and
The tetranacci numbers start with four predetermined terms, each term afterwards being the sum of the preceding four terms. The first few tetranacci numbers are:
- 0, 0, 0, 1, 1, 2, 4, 8, 15, 29, 56, 108, 208, 401, 773, 1490, 2872, 5536, 10671, 20569, 39648, 76424, 147312, 283953, 547337, … (sequence A000078 in OEIS)
The tetranacci constant is the ratio toward which adjacent tetranacci numbers tend. It is a root of the polynomial x4 − x3 − x2 − x − 1, approximately 1.92756, and also satisfies the equation x + x−4 = 2.
Pentanacci, hexanacci, and heptanacci numbers have been computed. The pentanacci numbers are:
- 0, 0, 0, 0, 1, 1, 2, 4, 8, 16, 31, 61, 120, 236, 464, 912, 1793, 3525, 6930, 13624, … (sequence A001591 in OEIS)
- 0, 0, 0, 0, 0, 1, 1, 2, 4, 8, 16, 32, 63, 125, 248, 492, 976, 1936, 3840, 7617, 15109, … (sequence A001592 in OEIS)
- 0, 0, 0, 0, 0, 0, 1, 1, 2, 4, 8, 16, 32, 64, 127, 253, 504, 1004, 2000, 3984, 7936, 15808, … (sequence A122189 in OEIS)
The limit of the ratio of successive terms of an n-nacci series tends to a root of the equation
An alternate recursive formula for the limit of ratio r of two consecutive n-nacci numbers can be expressed as .
The special case n=2 is the traditional Fibonacci series yielding the gold section .
The above formulas for the ratio hold even for n-nacci series generated from arbitrary numbers. The limit of this ratio is 2 as n increases. A 'polynacci' sequence, if one could be described, would after an infinite number of zeroes yield the sequence
- [..., 0, 0, 1,] 1, 2, 4, 8, 16, 32, …
which are simply powers of 2.
And the kth element of the n-nacci sequence is given by
where the outer brackets denote the nearest integer function and r is the n-nacci constant which is the root of near to 2.
A Coin Tossing Problem is related to the n-nacci sequence. The probability that no n consecutive tails will occur in m tosses of an idealized coin is . 
In analogy to its numerical counterpart, the Fibonacci word is defined by:
where + denotes the concatenation of two strings. The sequence of Fibonacci strings starts:
- b, a, ab, aba, abaab, abaababa, abaababaabaab, …
The length of each Fibonacci string is a Fibonacci number, and similarly there exists a corresponding Fibonacci string for each Fibonacci number.
If "a" and "b" represent two different materials or atomic bond lengths, the structure corresponding to a Fibonacci string is a Fibonacci quasicrystal, an aperiodic quasicrystal structure with unusual spectral properties.
The Fibonacci polynomials are another generalization of Fibonacci numbers.
The Padovan sequence is generated by the recurrence P(n) = P(n − 2) + P(n − 3).
A random Fibonacci sequence can be defined by tossing a coin for each position n of the sequence and taking F(n)=F(n−1)+F(n−2) if it lands heads and F(n)=F(n−1)−F(n−2) if it lands tails. Work by Furstenburg and Kesten guarantees that this sequence almost surely grows exponentially at a constant rate: the constant is independent of the coin tosses and was computed in 1999 by Divakar Viswanath. It is now known as Viswanath's constant.
A repfigit, or Keith number, is an integer such that, when its digits start a Fibonacci sequence with that number of digits, the original number is eventually reached. An example is 47, because the Fibonacci sequence starting with 4 and 7 (4,7,11,18,29,47) reaches 47. A repfigit can be a tribonacci sequence if there are 3 digits in the number, a tetranacci number if the number has four digits, etc. The first few repfigits are:
- 14, 19, 28, 47, 61, 75, 197, 742, 1104, 1537, 2208, 2580, 3684, 4788, 7385, 7647, 7909, … (sequence A007629 in OEIS)
Since the set of sequences satisfying the relation S(n) = S(n−1) + S(n−2) is closed under termwise addition and under termwise multiplication by a constant, it can be viewed as a vector space. Any such sequence is uniquely determined by a choice of two elements, so the vector space is two-dimensional. If we abbreviate such a sequence as (S(0), S(1)), the Fibonacci sequence F(n) = (0, 1) and the shifted Fibonacci sequence F(n−1) = (1, 0) are seen to form a canonical basis for this space, yielding the identity:
- S(n) = S(0)F(n−1) + S(1)F(n)
for all such sequences S. For example, if S is the Lucas sequence 2, 1, 3, 4, 7, 11…, then we obtain .