Definition
underlying: base asset of a interest-bearing token
interest-bearing token: IOU like cDAI, eUSDC
$x$: Underlying reserve in Pool
$y$: Zero-coupon bond reserve in Pool
Previous Work
Constant reserve: $V_{rsv}(x, y) = x + y$ , the sum value function.
Constant product: $V_{prod}(x, y) = \sqrt{xy}$, the product value function.