- xn = P xn−1 − Q xn−2
where P and Q are fixed integers. Any other sequence satisfying this recurrence relation can be represented as a linear combination of the Lucas sequences Un(P,Q) and Vn(P,Q).
More generally, Lucas sequences Un(P,Q) and Vn(P,Q) represent sequences of polynomials in P and Q with integer coefficients.
Famous examples of Lucas sequences include the Fibonacci numbers, Mersenne numbers, Pell numbers, Lucas numbers, Jacobsthal numbers, and a superset of Fermat numbers. Lucas sequences are named after the French mathematician Édouard Lucas.
Given two integer parameters P and Q, the Lucas sequences of the first kind Un(P,Q) and of the second kind Vn(P,Q) are defined by the recurrence relations:
It is not hard to show that for ,
Initial terms of Lucas sequences Un(P,Q) and Vn(P,Q) are given in the table:
The characteristic equation of the recurrence relation for Lucas sequences and is:
It has the discriminant and the roots:
Note that the sequence and the sequence also satisfy the recurrence relation. However these might not be integer sequences.
When , a and b are distinct and one quickly verifies that
It follows that the terms of Lucas sequences can be expressed in terms of a and b as follows
The case occurs exactly when for some integer S so that . In this case one easily finds that
Additional sequences having the same discriminant
If the Lucas sequences and have discriminant , then the sequences based on and where
have the same discriminant: .
|General||P = 1, Q = -1|
Among the consequences is that is a multiple of , i.e., the sequence is a divisibility sequence. This implies, in particular, that can be prime only when n is prime. Another consequence is an analog of exponentiation by squaring that allows fast computation of for large values of n. These facts are used in the Lucas–Lehmer primality test.
The Lucas sequences for some values of P and Q have specific names:
- Un(1,−1) : Fibonacci numbers
- Vn(1,−1) : Lucas numbers
- Un(2,−1) : Pell numbers
- Vn(2,−1) : Companion Pell numbers or Pell-Lucas numbers
- Un(1,−2) : Jacobsthal numbers
- Vn(1,−2) : Jacobsthal-Lucas numbers
- Un(3, 2) : Mersenne numbers 2n − 1
- Vn(3, 2) : Numbers of the form 2n + 1, which include the Fermat numbers (Yubuta 2001).
- Un(x,−1) : Fibonacci polynomials
- Vn(x,−1) : Lucas polynomials
- Un(x+1, x) : Repunits base x
- Vn(x+1, x) : xn + 1
Some Lucas sequences have entries in the On-Line Encyclopedia of Integer Sequences:
-1 3 A214733 1 -1 A000045 A000032 1 1 A128834 A087204 1 2 A107920 2 -1 A000129 A002203 2 1 A001477 2 2 A009545 A007395 2 3 A088137 2 4 A088138 2 5 A045873 3 -5 A015523 A072263 3 -4 A015521 A201455 3 -3 A030195 A172012 3 -2 A206776 3 -1 A006190 A006497 3 1 A001906 A005248 3 2 A000225 A000051 3 5 A190959 4 -3 A015530 A080042 4 -2 A090017 4 -1 A001076 A014448 4 1 A001353 A003500 4 2 A056236 4 3 A003462 A034472 4 4 A001787 5 -3 A015536 5 -2 A015535 5 -1 A087130 5 1 A003501 5 4 A002450 A052539
- Lucas sequences are used in probabilistic Lucas pseudoprime tests, which are part of the commonly used Baillie-PSW primality test.
- Lucas sequences are used in some primality proof methods, including the Lucas-Lehmer-Riesel test, and the N+1 and hybrid N-1/N+1 methods such as those in Brillhart-Lehmer-Selfridge 1975
- LUC is a public-key cryptosystem based on Lucas sequences that implements the analogs of ElGamal (LUCELG), Diffie-Hellman (LUCDIF), and RSA (LUCRSA). The encryption of the message in LUC is computed as a term of certain Lucas sequence, instead of using modular exponentiation as in RSA or Diffie-Hellman. However, a paper by Bleichenbacher et al. shows that many of the supposed security advantages of LUC over cryptosystems based on modular exponentiation are either not present, or not as substantial as claimed.
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- P. J. Smith, M. J. J. Lennon (1993). "LUC: A new public key system". Proceedings of the Ninth IFIP Int. Symp. on Computer Security: 103–117.
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