The Lucas numbers or Lucas series are an integer sequence named after the mathematician François Édouard Anatole Lucas, who studied both that sequence and the closely related Fibonacci numbers. Lucas numbers and Fibonacci numbers form complementary instances of Lucas sequences. The Lucas sequence has the same recursive relationship as the Fibonacci sequence, where each term is the sum of the two previous terms, but with different starting values. This produces a sequence where the ratios of successive terms approach the golden ratio, and in fact the terms themselves are roundings of integer powers of the golden ratio. The sequence also has a variety of relationships with the Fibonacci numbers, like the fact that adding any two Fibonacci numbers two terms apart in the Fibonacci sequence results in the Lucas number in between. The first few Lucas numbers are
Definition
Similar to the Fibonacci numbers, each Lucas number is defined to be the sum of its two immediate previous terms, thereby forming a Fibonacci integer sequence. The first two Lucas numbers are L0 = 2 and L1 = 1 as opposed to the first two Fibonacci numbers F0 = 0 and F1 = 1. Though closely related in definition, Lucas and Fibonacci numbers exhibit distinct properties. The Lucas numbers may thus be defined as follows: The sequence of the first twelve Lucas numbers is: All Fibonacci-like integer sequences appear in shifted form as a row of the Wythoff array; the Fibonacci sequence itself is the first row and the Lucas sequence is the second row. Also like all Fibonacci-like integer sequences, the ratio between two consecutive Lucas numbers converges to the golden ratio.
Extension to negative integers
Using Ln−2 = Ln − Ln−1, one can extend the Lucas numbers to negative integers to obtain a doubly infinite sequence: The formula for terms with negative indices in this sequence is
Their closed formula is given as: where is the golden ratio. Alternatively, as for the magnitude of the term is less than 1/2, is the closest integer to or, equivalently, the integer part of, also written as. Combining the above with Binet's formula, a formula for is obtained:
If Fn ≥ 5 is a Fibonacci number then no Lucas number is divisible by Fn. Ln is congruent to 1 mod n if n is prime, but some composite values of n also have this property. These are the Fibonacci pseudoprimes. Ln - Ln-4 is congruent to 0 mod 5.
Lucas primes
A Lucas prime is a Lucas number that is prime. The first few Lucas primes are The indices of these primes are If Ln is prime then n is 0, prime, or a power of 2. L2m is prime for m = 1, 2, 3, and 4 and no other known values of m.
Generating series
Let be the generating series of the Lucas numbers. By a direct computation, which can be rearranged as The partial fraction decomposition is given by where is the golden ratio and is its conjugate.
In the same way as Fibonacci polynomials are derived from the Fibonacci numbers, the Lucas polynomials Ln are a polynomial sequence derived from the Lucas numbers.
