Spirograph a hypocycloid. Spirograph Spirograph is a geometric drawing toy that produces mathematical roulette curves of the variety technically known as hypotrochoids and epitrochoids. It was developed by British engineer Denys Fisher and first sold in 1965. The name has been a registered trademark of Hasbro Inc. since 1998 following purchase of the company that had acquired the Denys Fisher company. The Spirograph brand was relaunched worldwide with original product configurations in 2013 by Kahootz Toys. In 1827 English architect and engineer Peter Hubert Desvignes developed his "Speiragraph", a machine to create elaborate spiral drawings, intended to prevent bank note forgeries.
Cone of curves extremal rays of the cone cannot accumulate. If in addition the variety formula_1 is defined over a field of characteristic 0, we have the following assertion, sometimes referred to as the Contraction Theorem: 3. Let formula_48 be an extremal face of the cone of curves on which formula_33 is negative. Then there is a unique morphism formula_50 to a projective variety "Z", such that formula_51 and an irreducible curve formula_8 in formula_1 is mapped to a point by formula_54 if and only if formula_55. (See also: contraction morphism). Cone of curves In mathematics, the cone of curves (sometimes the Kleiman-Mori