De Sitter and Einstein Willem de Sitter's interest in gravity was based on his work on celestial mechanics, in particular on the four big moons of Jupiter. His work on cosmology was based on the general theory of relativity of Albert Einstein. De Sitter published in 1917, on request of Arthur Eddington to inform the English astronomers, a series of four articles in The Observatory and the Monthly Notices of the Royal Astronomical Society. Einstein developed his own cosmological models, containing mass. De Sitter found a different solution and described a universe without mass. Einstein could not accept De Sitter's model and they 'fought out' two controversies in their correspondence. In the end Einstein had to confess De Sitter was mainly right in his criticisms. In 1932 Einstein and De Sitter published an article on a new model, the so-called Einstein-De Sitter Model of the universe. So, De Sitter was able to do fundamental work in classical celestial mechanics as well as in the new cosmological theories.

No abstract

No abstract

In 1960, a mathematical journal for high school students was founded in the Netherlands, an idea of the Dutch mathematician Hans Freudenthal. Its name: Pythagoras. The first two editors were the mathematicians Gerrit Krooshof and Bruno Ernst, the latter being a personal friend and biographer of the Dutch graphic artist Maurits Escher. The "impossible figures" of Escher have been an ever-recurring subject over the years. In 2011, the editors compiled problems, riddles, games, and articles from fifty years of Pythagoras into a book, to celebrate half a century of the magazine's existence. Now, four years later, an English translation of this anniversary book appears, supplemented with some additional articles from the magazine. Bruno Ernst compared mathematics to a luxuriant garden: high in the trees are the brilliant mathematicians, who have been toiling for years to reach the top. But down on the grass, within everybody's reach, there are beautiful flowers to pick as well. With many subjects in this book, everybody can make a start to pick them, especially in the first two chapters. Some puzzles belong to mathematical folklore. The greater part however stems from the minds of the many editors over the years. Apart from puzzles and games there are articles on art, geometry, and numbers. But beware, there is also a lot of more challenging mathematics for the connoisseur. Half a Century of Pythagoras Magazine is a rich treasure for everybody who enjoys the beauty of mathematics.

No abstract