In a world of no corporate taxes if the use of leverage does not change the value of the levered firm relative to the unlevered firm is

known as:

MM Proposition I that the market value of the firm is invariant to the capital structure.

MM Proposition III that the cost of stock is less than the cost of debt.

MM Proposition II that the cost of equity is always constant.

MM Proposition I that leverage is invariant to market value.

MM Proposition III that there is no risk associated with leverage in a no tax world.