# Euler number

{{#invoke:Hatnote|hatnote}} In number theory, the Euler numbers are a sequence En of integers (sequence A122045 in OEIS) defined by the following Taylor series expansion:

${\displaystyle {\frac {1}{\cosh t}}={\frac {2}{e^{t}+e^{-t}}}=\sum _{n=0}^{\infty }{\frac {E_{n}}{n!}}\cdot t^{n}\!}$

where cosh t is the hyperbolic cosine. The Euler numbers appear as a special value of the Euler polynomials.

The odd-indexed Euler numbers are all zero. The even-indexed ones (sequence A028296 in OEIS) have alternating signs. Some values are:

E0 = 1
E2 = −1
E4 = 5
E6 = −61
E8 = 1,385
E10 = −50,521
E12 = 2,702,765
E14 = −199,360,981
E16 = 19,391,512,145
E18 = −2,404,879,675,441

Some authors re-index the sequence in order to omit the odd-numbered Euler numbers with value zero, and/or change all signs to positive. This encyclopedia adheres to the convention adopted above.

The Euler numbers appear in the Taylor series expansions of the secant and hyperbolic secant functions. The latter is the function in the definition. They also occur in combinatorics, specifically when counting the number of alternating permutations of a set with an even number of elements.

## Explicit formulas

### Iterated sum

An explicit formula for Euler numbers is given by:[1]

${\displaystyle E_{2n}=i\sum _{k=1}^{2n+1}\sum _{j=0}^{k}{k \choose j}{\frac {(-1)^{j}(k-2j)^{2n+1}}{2^{k}i^{k}k}}}$

where i denotes the imaginary unit with i2=−1.

### Sum over partitions

The Euler number E2n can be expressed as a sum over the even partitions of 2n,[2]

${\displaystyle E_{2n}=(2n)!\sum _{0\leq k_{1},\ldots ,k_{n}\leq n}~\left({\begin{array}{c}K\\k_{1},\ldots ,k_{n}\end{array}}\right)\delta _{n,\sum mk_{m}}\left({\frac {-1~}{2!}}\right)^{k_{1}}\left({\frac {-1~}{4!}}\right)^{k_{2}}\cdots \left({\frac {-1~}{(2n)!}}\right)^{k_{n}},}$

as well as a sum over the odd partitions of 2n − 1,[3]

${\displaystyle E_{2n}=(-1)^{n-1}(2n-1)!\sum _{0\leq k_{1},\ldots ,k_{n}\leq 2n-1}\left({\begin{array}{c}K\\k_{1},\ldots ,k_{n}\end{array}}\right)\delta _{2n-1,\sum (2m-1)k_{m}}\left({\frac {-1~}{1!}}\right)^{k_{1}}\left({\frac {1}{3!}}\right)^{k_{2}}\cdots \left({\frac {(-1)^{n}}{(2n-1)!}}\right)^{k_{n}},}$

where in both cases ${\displaystyle K=k_{1}+\cdots +k_{n}}$ and

${\displaystyle \left({\begin{array}{c}K\\k_{1},\ldots ,k_{n}\end{array}}\right)\equiv {\frac {K!}{k_{1}!\cdots k_{n}!}}}$

is a multinomial coefficient. The Kronecker delta's in the above formulas restrict the sums over the k's to ${\displaystyle 2k_{1}+4k_{2}+\cdots +2nk_{n}=2n}$ and to ${\displaystyle k_{1}+3k_{2}+\cdots +(2n-1)k_{n}=2n-1}$, respectively.

As an example,

{\displaystyle {\begin{aligned}E_{10}&=10!\left(-{\frac {1}{10!}}+{\frac {2}{2!8!}}+{\frac {2}{4!6!}}-{\frac {3}{2!^{2}6!}}-{\frac {3}{2!4!^{2}}}+{\frac {4}{2!^{3}4!}}-{\frac {1}{2!^{5}}}\right)\\&=9!\left(-{\frac {1}{9!}}+{\frac {3}{1!^{2}7!}}+{\frac {6}{1!3!5!}}+{\frac {1}{3!^{3}}}-{\frac {5}{1!^{4}5!}}-{\frac {10}{1!^{3}3!^{2}}}+{\frac {7}{1!^{6}3!}}-{\frac {1}{1!^{9}}}\right)\\&=-50,521.\end{aligned}}}

### Determinant

E2n is also given by the determinant

{\displaystyle {\begin{aligned}E_{2n}&=(-1)^{n}(2n)!~{\begin{vmatrix}{\frac {1}{2!}}&1&~&~&~\\{\frac {1}{4!}}&{\frac {1}{2!}}&1&~&~\\\vdots &~&\ddots ~~&\ddots ~~&~\\{\frac {1}{(2n-2)!}}&{\frac {1}{(2n-4)!}}&~&{\frac {1}{2!}}&1\\{\frac {1}{(2n)!}}&{\frac {1}{(2n-2)!}}&\cdots &{\frac {1}{4!}}&{\frac {1}{2!}}\end{vmatrix}}.\end{aligned}}}

## Asymptotic approximation

The Euler numbers grow quite rapidly for large indices as they have the following lower bound

${\displaystyle |E_{2n}|>8{\sqrt {\frac {n}{\pi }}}\left({\frac {4n}{\pi e}}\right)^{2n}.}$

## Euler zigzag numbers

1, 1, 1, 2, 5, 16, 61, 272, 1385, 7936, 50521, 353792, 2702765, 22368256, 199360981, 1903757312, 19391512145, 209865342976, 2404879675441, 29088885112832, ... (sequence A000111 in OEIS)

## Generalized Euler numbers

One of the generalizations of Euler numbers is Poly-Euler numbers which plays an important role to multiple Euler-Zeta function