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expn
Exponential integral En.SYNOPSIS:
int n;
double x, y, expn();
y = expn(n, x);
DESCRIPTION:
Evaluates the exponential integral.
inf.
-
| | -xt
| e
E (x) = | ---- dt.
n | n
| | t
-
1
Both n and x must be nonnegative.
The routine employs either a power series, a continued fraction, or an asymptotic formula depending on the relative values of n and x.
ACCURACY:
Relative error:
arithmetic domain # trials peak rms
DEC 0, 30 5000 2.0e-16 4.6e-17
IEEE 0, 30 10000 1.7e-15 3.6e-16