exp-int ( x -- y )

Vocabulary

math.analysis

Inputs

x

a number

Outputs

y

a number

Word description

Exponential integral function.

Notes

Works only for real values of x and is accurate to 7 decimal places.

Definition

USING: kernel math math.functions ;

IN: math.analysis

: exp-int ( x -- y )
    dup 1.0 < [
        dup 0.00107857 * 0.00976004 - over * 0.05519968 + over *
        0.24991055 - over * 0.99999193 + over * 0.57721566 -
        swap log -
    ] [
        dup 8.5733287401 + over * 18.059016973 + over *
        8.6347608925 + over * 0.2677737343 +
        over dup 9.5733223454 + over * 25.6329561486 + over *
        21.0996530827 + over * 3.9584969228 + nip / over /
        swap -1.0 * e^ *
    ] if ;