Denys Fisher Denys Fisher Denys Fisher (11 May 1918 – 17 September 2002) was an English engineer who invented the spirograph toy and created the company Denys Fisher Toys. He left Leeds University to join the family firm, Kingfisher (Lubrication) Ltd. In 1960 he left the firm to set up his own company, Denys Fisher Engineering, in Leeds. In 1961 the company won a contract with NATO to supply springs and precision components for its 20 mm cannon. Between 1962 and 1964 he developed various drawing machines from Meccano pieces, eventually producing a prototype Spirograph. Patented in 16 countries, it went on
Cone of curves Cone of curves In mathematics, the cone of curves (sometimes the Kleiman-Mori cone) of an algebraic variety formula_1 is a combinatorial invariant of much importance to the birational geometry of formula_1. Let formula_1 be a proper variety. By definition, a (real) "1-cycle" on formula_1 is a formal linear combination formula_5 of irreducible, reduced and proper curves formula_6, with coefficients formula_7. "Numerical equivalence" of 1-cycles is defined by intersections: two 1-cycles formula_8 and formula_9 are numerically equivalent if formula_10 for every Cartier divisor formula_11 on formula_1. Denote the real vector space of 1-cycles modulo numerical equivalence by formula_13. We define the