In 1956, the Dutch graphic artist M.C. Escher made an unusual lithograph with the title `Print Gallery‘. It shows a young man viewing a print in an exhibition gallery. Amongst the buildings depicted on the print, he sees paradoxically the very same gallery that he is standing in. A lot is known about the way in which Escher made his lithograph. It is not nearly as well known that it contains a hidden `Droste effect‘, or infinite repetition; but this is brought to light by a mathematical analysis of the studies used by Escher. On the basis of this discovery, a team of mathematicians at Leiden produced a series of hallucinating computer animations. These show, among others, what happens inside the mysterious spot in the middle of the lithograph that Escher left blank.

**Hendrik Lenstra** received his doctorate from the University of Amsterdam in 1977 and became a professor there in 1978. In 1987 he was appointed to the faculty of the University of California, Berkeley; starting in 1998, he divided his time between Berkeley and the University of Leiden, until 2003, when he retired from Berkeley to take a full-time position at Leiden. He was awarded the Spinoza prize in 1998, and on 24 April 2009 he was made a Knight of the Order of the Netherlands Lion. Lenstra has worked principally in computational number theory and is well known as the discoverer of the elliptic curve factorization method and a co-discoverer of the Lenstra–Lenstra–Lovász lattice basis reduction algorithm.

Hendrik Lenstra is currently Hirzebruch Visiting Professor at the Max Planck Institute for Mathematics in Bonn.