[sf.cmath.expint]

# 29 Numerics library [[numerics]](./#numerics)

## 29.7 Mathematical functions for floating-point types [[c.math]](c.math#sf.cmath.expint)

### 29.7.6 Mathematical special functions [[sf.cmath]](sf.cmath#expint)

#### 29.7.6.15 Exponential integral [sf.cmath.expint]

[🔗](#lib:expint)

`floating-point-type expint(floating-point-type x);
float        expintf(float x);
long double  expintl(long double x);
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L10525)

*Effects*: These functions compute the exponential integral
of their respective argumentsx[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L10531)

*Returns*: Ei(x),
where Ei is given by Formula [29.35](#eq:sf.cmath.expint) andx is x[.](#2.sentence-1)

Ei(x)=−∫∞−xe−ttdt(29.35)