public double lowLimNeg() {
        double[] helpArray = new double[equationCoef.length];
        for (int i = 0; i < equationCoef.length; i++) {
            if (equationPower[i] % 2 == 0) {
                helpArray[i] =  equationCoef[i];
            } else {
                helpArray[i] = - equationCoef[i];
            }
        }
        
        // First polinom coef cshoul be positive
        if (helpArray[0] < 0) {
            for (int j = 0; j < helpArray.length; j++) {
                helpArray[j] = - helpArray[j];
            }
        } 
        double lowLimN = - this.upLimPlus(helpArray);
        return lowLimN;
    }