清空筛选条件
清空筛选条件

Difference in MATLAB function evaluation

2 次查看（过去 30 天）
GKH
GKH 2017-1-31
评论： Star Strider 2017-1-31
According to the properties of gamma function, for any x > 0
But when I try to use it in MATLAB, it shows a very large error
gamma(150) - 149*gamma(149)
ans =
-1.5240e+247
Can anybody explain the reason for this? I need it for an approximation algorithm I am trying to develop.
Thanks in advance.
  1 个评论
Star Strider
Star Strider 2017-1-31
FWIW, I get similar results for these (in R2016b):
n = 149;
q1 = gamma(n+1) - n*gamma(n);
q2 = gamma(n+1) - factorial(n);
q3 = gamma(n+1) - prod(1:n);
q1 =
-15.2402e+246
q2 =
-819.3656e+243
q3 =
-819.3656e+243
I hope this is a problem with my understanding of the gamma function, and not a problem with its implementation in MATLAB.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Honglei Chen
Honglei Chen 2017-1-31
For your use case, 150! is a very large number so the numerical precision issue starts to kick in. I'd suggest you to use gammaln instead, for example, you can do
gammaln(150)-(log(149)+gammaln(149))
and the result is indeed 0.
HTH.

更多回答（1 个）

Jan
Jan 2017-1-31
编辑：Jan 2017-1-31
While the absolute error is huge, the relative error is tiny:
(gamma(150) - 149*gamma(149)) / gamma(150)
>> -3.9868e-014
This is the expected error caused by using the IEEE754 double format. This stores about 16 valid digits.
If an approxmation algorithm is sensitive for this difference, it is instable and of limited use.

类别

在 Help Center 和 File Exchange 中查找有关 Gamma Functions 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by