Genocchi number

Template:Short description In mathematics, the Genocchi numbers G_n, named after Angelo Genocchi, are a sequence of integers that satisfy the relation

$\frac{2t}{1+e^t}=\sum _{n=0}^{\mathrm{\infty }}{G}_n\frac{t^n}{n!}$

The first few Genocchi numbers are 0, 1, −1, 0, 1, 0, −3, 0, 17 Template:OEIS, see Template:OEIS2C.

Properties

• The generating function definition of the Genocchi numbers implies that they are rational numbers. In fact, G_2n+1 = 0 for n ≥ 1 and (−1)ⁿG_2n is an odd positive integer.
• Genocchi numbers G_n are related to Bernoulli numbers B_n by the formula

$G_n=2(1-2^n)B_n.$

Combinatorial interpretations

The exponential generating function for the signed even Genocchi numbers (−1)ⁿG_2n is

$t\tan \left(\frac{t}{2}\right)=\sum _{n\ge 1}(-1{)}^n{G}_{2n}\frac{t^{2n}}{(2n)!}$

They enumerate the following objects:

• Permutations in S_2n−1 with descents after the even numbers and ascents after the odd numbers.
• Permutations π in S_2n−2 with 1 ≤ π(2i−1) ≤ 2n−2i and 2n−2i ≤ π(2i) ≤ 2n−2.
• Pairs (a₁,...,a_n−1) and (b₁,...,b_n−1) such that a_i and b_i are between 1 and i and every k between 1 and n−1 occurs at least once among the a_i's and b_i's.
• Reverse alternating permutations a₁ < a₂ > a₃ < a₄ >...>a_2n−1 of [2n−1] whose inversion table has only even entries.

Primes

The only known prime numbers which occur in the Genocchi sequence are 17, at n = 8, and -3, at n = 6 (depending on how primes are defined). It has been proven that no other primes occur in the sequence

See also

• Euler number

References

• Template:MathWorld
• Richard P. Stanley (1999). Enumerative Combinatorics, Volume 2, Exercise 5.8. Cambridge University Press. Template:ISBN
• Gérard Viennot, Interprétations combinatoires des nombres d'Euler et de Genocchi, Seminaire de Théorie des Nombres de Bordeaux, Volume 11 (1981-1982)
• Serkan Araci, Mehmet Acikgoz, Erdoğan Şen, Some New Identities of Genocchi Numbers and Polynomials