VGAM (version 0.9-6)

expint: The Exponential Integral and Variants

Description

Computes the exponential integral $Ei(x)$ for real values, as well as $\exp(-x) \times Ei(x)$ and $E_1(x)$.

Usage

expint(x)
expexpint(x)
expint.E1(x)

Arguments

x

Numeric. Ideally a vector of positive reals.

Value

• Function expint(x) returns $Ei(x)$, function expexpint(x) returns $\exp(-x) \times Ei(x)$, function expint.E1(x) returns $E_1(x)$.

Details

The exponential integral $Ei(x)$ function is the integral of $exp(t) / t$ from 0 to $x$, for positive real $x$. The function $E_1(x)$ is the integral of $exp(-t) / t$ from $x$ to infinity, for positive real $x$.

References

http://www.netlib.org/specfun/ei.

See Also

log, exp.

Examples

par(mfrow = c(2, 2))
curve(expint, 0.01, 2, xlim = c(0, 2), ylim = c(-3, 5),
      las = 1, col = "orange")
abline(v = (-3):5, h = (-4):5, lwd = 2, lty = "dotted", col = "gray")
abline(h = 0, v = 0, lty = "dashed", col = "blue")

curve(expexpint, 0.01, 2, xlim = c(0, 2), ylim = c(-3, 2),
      las = 1, col = "orange")
abline(v = (-3):2, h = (-4):5, lwd = 2, lty = "dotted", col = "gray")
abline(h = 0, v = 0, lty = "dashed", col = "blue")

curve(expint.E1, 0.01, 2, xlim = c(0, 2), ylim = c(0, 5),
      las = 1, col = "orange")
abline(v = (-3):2, h = (-4):5, lwd = 2, lty = "dotted", col = "gray")
abline(h = 0, v = 0, lty = "dashed", col = "blue")