It was in January 1990. Finally, just two months after the Berlin Wall had fallen, I had the opportunity to spend a few days at the Mittag-Leffler Institute in Djursholm, a small town just northeast of Stockholm. The palatial villa that today houses the Institute is the former home of Gösta Mittag-Leffler (1846–1927), and on entering its doorway I felt as if I had taken a step back into the world in which he lived. For me, the Institute's single greatest attraction lay in its archival holdings, and particularly the extensive correspondence that linked Mittag-Leffler with many of the era's leading mathematicians. A former student of Karl Weierstrass (1815–1897), Mittag-Leffler sought to preserve everything he could get his hands on from the master's estate. Thanks to his efforts, many letters, manuscripts, and other documents associated with Weierstrass have survived today. In Berlin, on the other hand, where Weierstrass passed his entire scientific career, there is no corresponding collection of materials.