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A general description is here.
Since R 2.10.0, ‘z^n’ was computed incorrectly for ‘n >= 3’ and ‘n <= -3’ on platforms for which the macro ‘HAVE_C99_COMPLEX’ was not defined. Such platforms have been very rare, but now include x86_64 Linux with ‘gcc 4.5.x’ at the default ‘-O2’ optimization (but not at ‘-O’).