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1 − 1 + 2 − 6 + 24 − 120 + ⋯.
数学上,发散级数:
![{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}k!}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e12d0c02d8e5724065a8ddaad6fb08c02a486c3a)
是被欧拉首次研究,他应用重求和方法给级数赋予一个有限的值[1]。此级数是被交替加减的阶乘之总和。要给发散级数赋值,其中一个方法是用博雷尔和,其型式上写成:
![{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}k!=\sum _{k=0}^{\infty }(-1)^{k}\int _{0}^{\infty }x^{k}\exp(-x)\,dx}](https://wikimedia.org/api/rest_v1/media/math/render/svg/75fe6fd2d1c31d460e14d4bae0ba49ce844a6096)
若我们对总和和积分进行转乘(忽略两者其实都是不收敛的),将得到:
![{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}k!=\int _{0}^{\infty }\left[\sum _{k=0}^{\infty }(-x)^{k}\right]\exp(-x)\,dx}](https://wikimedia.org/api/rest_v1/media/math/render/svg/90721db81a4e9040c4c959858db09342139fb25c)
在中括号中的总和收敛,并等于1/(1 + x),若x < 1。若我们继续对所有实数x分析1/(1 + x),可以得到收敛积分的总和:
![{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}k!=\int _{0}^{\infty }{\frac {\exp(-x)}{1+x))\,dx=eE_{1}(1)\approx 0.596347362323194074341078499369\ldots }](https://wikimedia.org/api/rest_v1/media/math/render/svg/58802c20fc9a50f754036f46c966e3d202fb2d9d)
此处的
是指数积分。这是根据博雷尔和对级数的定义。
结果
若k为前十个值,其结果如下:
k |
增量 计算 |
增量 |
结果
|
0 |
1 · 0! = 1 · 1 |
1 |
1
|
1 |
−1 · 1 |
−1 |
0
|
2 |
1 · 2 · 1 |
2 |
2
|
3 |
−1 · 3 · 2 · 1 |
−6 |
−4
|
4 |
1 · 4 · 3 · 2 · 1 |
24 |
20
|
5 |
−1 · 5 · 4 · 3 · 2 · 1 |
−120 |
−100
|
6 |
1 · 6 · 5 · 4 · 3 · 2 · 1 |
720 |
620
|
7 |
−1 · 7 · 6 · 5 · 4 · 3 · 2 · 1 |
−5040 |
−4420
|
8 |
1 · 8 · 7 · 6 · 5 · 4 · 3 · 2 · 1 |
40320 |
35900
|
9 |
−1 · 9 · 8 · 7 · 6 · 5 · 4 · 3 · 2 · 1 |
−362880 |
−326980
|
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