ExpIntegralEi[z] == ExpIntegralEi[x] + Pi I - I Pi Floor[(Pi + Arg[z])/(2 Pi)] + 2 I Pi Floor[Arg[z - x]/(2 Pi)] + Sum[((-1)^(k - 1)/(x^k k) + (x^(1 - k)/k!) HypergeometricPFQRegularized[ {1, 1}, {2, 2 - k}, x]) (z - x)^k, {k, 1, Infinity}] /; Element[x, Reals] && x < 0

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2007-05-02