Child prodigy in both music and mathematics who earned fame after solving decades-old problems in functional analysis.
He came in first at the 1956 Swedish young pianists competition when he was 11, later winning it again in 1961.
He solved mathematics' basis problem, approximation problem, and invariant subspace problem for Banach spaces, thus influencing generations of functional analysis and operator-theory researchers and impacting computer science and number theory.
His interest in piano came when he was seven and his younger brother Hans begged their parents to begin them on piano lessons.
He was a competitor at the first annual International Piano Competition for Outstanding Amateurs in 1999, which was hosted by the Van Cliburn Foundation.