Error analysis in a uniform asymptotic expansion for the generalised exponential integral

[摘要] Uniform asymptotic expansions are derived for the generalised exponential integral E-p(z), where both p and z are complex. These are derived by examining the differential equation satisfied by E-p(z), an equation which possesses a double turning point at z/p = -1. The expansions, which involve the complementary error function, together approximate E-p(z) as \p\ --> infinity, uniformly for all non-zero complex z satisfying 0 less than or equal to arg(z/p) less than or equal to 2 pi. The error terms associated with the truncated expansions are shown to be solutions of inhomogeneous differential equations, and from these explicit and realistic bounds are derived. By employing the Maximum-Modulus Theorem the bounds are then simplified to make them more conducive to numerical evaluation.

[发布日期] 1997-04-14

[关键词] generalised exponential integral;incomplete gamma functions;turning point theory;uniform asymptotic expansion;error function