Per H. Enflo (Swedish: [ˈpæːr ˈěːnfluː]; born 20 May 1944) is a Swedish mathematician working primarily in functional analysis, a field in which he solved problems that had been considered fundamental. Three of these problems had been open for more than forty years: * The basis problem and the approximation problem and later * the invariant subspace problem for Banach spaces. Enflo is also a concert pianist.