question: public static double expm1 ( double value ) { if ( USE_JDK_MATH ) { return STRICT_MATH ? StrictMath . expm1 ( value ) : Math . expm1 ( value ) ; } if ( Math . abs ( value ) < EXP_LO_DISTANCE_TO_ZERO ) { int i = ( int ) ( value * EXP_LO_INDEXING ) ; double delta = value - i * ( 1.0 / EXP_LO_INDEXING ) ; return expLoPosTab [ i + EXP_LO_TAB_MID_INDEX ] * ( expLoNegTab [ i + EXP_LO_TAB_MID_INDEX ] + delta * ( 1 + delta * ( 1.0 / 2 + delta * ( 1.0 / 6 + delta * ( 1.0 / 24 + delta * ( 1.0 / 120 ) ) ) ) ) ) ; } else { return FastMath . exp ( value ) - 1 ; } }
target:  Much more accurate than exp(value)-1, for values close to zero.