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C:\>perl -MMath::MPFR=":mpfr" -le "$op = Math::MPFR->new(-9117 ** 3); +Rmpfr_cbrt($op, $op, MPFR_RNDN); print $op;" -9.117e3
C:\>perl -MMath::MPFR=":mpfr" -le "$op = Math::MPFR->new(-9117 ** 3); +Rmpfr_rootn_ui($op, $op, 3, MPFR_RNDN); print $op;" -9.117e3