Taivo Arak is an Estonian politician and a member of the Estonian Centre Party (Eesti Keskerakond). He has been involved in local and national politics in Estonia and may have held various positions within the political landscape. For the latest information, including his current role or contributions, it’s best to consult recent sources or news articles, as my data only goes up to October 2023, and there might have been developments since then.

The Movable Book Society (MBS) is an organization dedicated to the appreciation and preservation of movable books, which include pop-up books, pop-up cards, and other specialty books that incorporate three-dimensional elements and mechanical features. Founded in 1993, the society connects enthusiasts, collectors, artists, and publishers with a shared interest in this unique form of literature and art. The MBS provides a platform for members to share information, resources, and techniques related to movable books.

Waldo Hunt is not a widely recognized term or concept, so there might be some ambiguity in your question. However, if you are referring to "Waldo Hunt," it may imply a search for a character named Waldo, popularized in the children's book series "Where's Waldo?" by Martin Handford, where readers are tasked with finding Waldo in various crowded illustrations.

"Between the Folds" is a documentary film released in 2008 that explores the world of contemporary origami and its intersection with art, mathematics, and science. Directed by Vanessa Gould, the film features interviews with various origami artists, scientists, and mathematicians who discuss both the aesthetic and utilitarian aspects of paper folding.

Kusudama is a traditional Japanese paper craft that involves creating a spherical model by assembling multiple folded paper pieces, often using origami techniques. The word "kusudama" translates to "medicine ball" in Japanese, as these decorative spheres were historically used to hold herbal medicines or as a way to promote health and wellness. Typically, a kusudama is made by folding several identical paper units, which are then assembled and glued or stitched together to form a 3D shape.

OrigamiUSA is a nonprofit organization dedicated to promoting the art and craft of origami, which involves the folding of paper into intricate designs and shapes. Founded in 1980, OrigamiUSA serves as a hub for origami enthusiasts, providing resources such as educational materials, workshops, and an annual convention where people of all skill levels can come together to learn, share, and celebrate their love for origami.

Rigid origami is a branch of origami that focuses on the folding of flat materials into three-dimensional shapes while maintaining the rigidity of the folded structure. Unlike traditional origami, where paper can be flexibly bent and manipulated, rigid origami relies on specific types of folds that allow a structure to maintain its shape without any deformation.

Differential forms on a Riemann surface are a fundamental concept in the field of complex geometry and algebraic geometry, and they provide a powerful language for analyzing the geometry of Riemann surfaces. A **Riemann surface** is a one-dimensional complex manifold, which can be thought of as a "smoothly varying" collection of complex charts that are compatible with one another.

A Hurwitz surface is a specific type of mathematical object in the field of algebraic geometry and topology. It is a smooth (or complex) surface that arises in the study of branched covers of Riemann surfaces. More specifically, Hurwitz surfaces are associated with the study of coverings of the Riemann sphere (the complex projective line) and are tied to the Hurwitz problem, which deals with the enumeration of branched covers of a surface.

A constructive dilemma is a valid logical argument form that involves a disjunction (an "either-or" statement) followed by conditional statements leading to a conclusion. It is used in propositional logic and typically follows a specific structure. The general form of a constructive dilemma can be expressed as follows: 1. \( P \lor Q \) (Either P or Q is true) 2. \( P \rightarrow R \) (If P is true, then R is true) 3.

Double negation is a logical principle stating that a proposition that is negated twice is equivalent to the proposition itself. In simpler terms, if you say "not not P," you are effectively affirming P. In formal logic, if "P" is a statement, then the double negation can be expressed as: ¬(¬P) ≡ P This principle is used in various fields, including mathematics, philosophy, and computer science.

The term "hexagon" can refer to a couple of different concepts depending on the context: 1. **Geometric Shape**: A hexagon is a polygon with six sides and six angles. In a regular hexagon, all sides are of equal length and all interior angles are equal, measuring 120 degrees each. The shape can be found in various natural and man-made structures, such as honeycomb patterns in beehives.

In logic, "exportation" is a valid rule of inference that deals with implications. It states that if you have a conditional statement of the form: 1.

In logic, a **tautology** is a statement or formula that is true in every possible interpretation, regardless of the truth values of its components. In other words, it is a logical expression that cannot be false. Tautologies are important in propositional logic and are often used as the basis for proving other statements. One common example of a tautology is the expression \( p \lor \neg p \) (where \( p \) is any proposition).

Refinement types are a type system feature that extends traditional type systems by allowing types to express more specific properties or constraints about values. They enable programmers to specify not just what type a value is, but also certain predicates that must hold true for values of that type. In a typical type system, a type like `Integer` simply describes integers without any additional constraints. Refinement types allow for the expression of constraints like "positive integers" or "even integers".

Jacques Herbrand was a French mathematician and logician, known for his significant contributions to mathematical logic, particularly in the areas of proof theory, model theory, and the foundations of mathematics. He was born on December 20, 1908, and died tragically young at the age of 27 in a car accident in 1931. Herbrand is especially recognized for Herbrand's theorem and Herbrand's universes, which are crucial in the context of first-order logic.

In music theory, a **pitch class** refers to a group of all pitches that are a whole number of octaves apart from each other. In other words, all the notes that have the same name, regardless of which octave they are in, belong to the same pitch class. For example, the pitch classes for C include all Cs, such as C4 (middle C), C5, C3, and so on. Each pitch class can be represented by a note name (e.

Endophora is a linguistic term that refers to a type of reference where a word or phrase relies on something mentioned within the same context, particularly within a text or discourse. It contrasts with exophora, which refers to references that draw on external contexts or knowledge outside of the discourse. In endophoric references, terms such as pronouns or definite descriptions refer back to previously mentioned entities or ideas within the same text. For example, in the sentence "The cat is on the roof.

X-FEN, or Extended Forsyth-Edwards Notation, is a notation system used to represent the state of a chess game, similar to the standard Forsyth-Edwards Notation (FEN).

Chinook is a computer program designed to play the game of checkers (draughts). It is particularly notable for being one of the first programs to reach a level of play that could effectively compete against human experts.