evaluates an approximation to the modified Bessel function of the first kind , where the order =-3, -2, -1, 1, 2 or 3 and x is real and positive. For positive orders it may also be called with x=0, since when > 0. For negative orders the formula
is used prior to multiplication by the scale factor .
For more information please review the s18ecc function in the NAG document.
x>0.0 when nu<0,
x???0.0 when nu>0.