Euler number

<math>\frac{2}{\exp (t) + \exp (-t) } = \sum_{n=0}^{\infin} \frac{E_n}{n!} \cdot t^n</math>

(Note that e, the base of the natural logarithm, is also occasionally called Euler's number, as is the Euler characteristic.)

The odd-indexed Euler numbers are all zero. The even-indexed ones 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 expansion of the secant trigonometric function, and they also occur in combinatorics.