Robert A. Mundell, a Nobel Prize-winning economist whose theorizing opened the door to understanding the workings of global finance and the modern-day international economy, while his more iconoclastic views on economic policy fostered the creation of the euro and the adoption of the tax-cutting approach known as supply-side economics, died on Sunday at his home, a Renaissance-era palazzo that he and his wife restored, near Siena, Italy. He was 88.