Rational Chebyshev approximations for the exponential integral 𝐸₁(π‘₯)

Abstract
Rational Chebyshev approximations are presented for the exponential integral E 1 ( x ) {E_1}(x) in the intervals ( 0 , 1 ] (0,1] , [ 1 , 4 ] [1,4] , and [ 4 , ∞ ) [4,\infty ) with maximal relative errors ranging down to 10 βˆ’ 21 {10^{ - 21}} . 25 S 25S coefficients are also given for a continued-fraction expansion for small X X .

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