Statement 1 The effective convexity of a putable bond cannot be less than that of an otherwise identical option-free bond.
Statement 2 The effective convexity of a callable bond can be negative in some circumstances, but the effective convexity of a putable bond is always positive.
Statement 3 The effective duration of a callable bond cannot be greater than that of an otherwise identical option-free bond, and the effective duration of a putable bond cannot be less than that of the option-free bond.
S1说putable bond的凸度要大于等于不含权债券，这难度不是正确的吗？上课老师说了，利率上升时，putable bond变得更凸，而下降的时候，两者一样。
