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A Messier marathon is an attempt, usually organized by amateur astronomers, to find as many Messier objects as possible during one night. The Messier catalogue was compiled by French astronomer Charles Messier during the late 18th century and consists of 110 relatively bright deep-sky objects (galaxies, nebulae, and star clusters).

The friendship paradox is the phenomenon first observed by the sociologist Scott L. Feld in 1991 that most people have fewer friends than their friends have, on average. It can be explained as a form of sampling bias in which people with greater numbers of friends have an increased likelihood of being observed among one's own friends. In contradiction to this, most people believe that they have more friends than their friends have.

The same observation can be applied more generally to social networks defined by other relations than friendship: for instance, most people's sexual partners have had (on the average) a greater number of sexual partners than they have.

In condensed matter physics, Hofstadter's butterfly describes the spectral properties of non-interacting two dimensional electrons in a magnetic field. The fractal, self-similar, nature of the spectrum was discovered in the 1976 Ph.D. work of Douglas Hofstadter and is one of the early examples of computer graphics. The name reflects the visual resemblance of the figure on the right to a swarm of butterflies flying to infinity.

The Hofstadter butterfly plays an important role in the theory of the integer quantum Hall effect, and D.J. Thouless has been awarded the Nobel prize in physics in 2016 for the discovery that the wings of the butterfly are characterized by Chern integers, the quantized Hall conductances discovered in 1980 by Klaus von Klitzing for which he has been awarded the Nobel prize in 1985. The colors in the diagram reflect the different Chern numbers.

The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.

The Chandra–Toueg consensus algorithm, published by Tushar Deepak Chandra and Sam Toueg in 1996, is an algorithm for solving consensus in a network of unreliable processes equipped with an eventually strong failure detector. The failure detector is an abstract version of timeouts; it signals to each process when other processes may have crashed. An eventually strong failure detector is one that never identifies some specific non-faulty process as having failed after some initial period of confusion, and, at the same time, eventually identifies all faulty processes as failed (where a faulty process is a process which eventually fails or crashes and a non-faulty process never fails). The Chandra–Toueg consensus algorithm assumes that the number of faulty processes, denoted by f, is less than n/2 (i.e. the minority), i.e. it assumes f < n/2, where n is the total number of processes.

El Paquete Semanal ("The Weekly Package") or El Paquete is a one terabyte collection of digital material distributed since around 2008 on the underground market in Cuba as a substitute for broadband Internet. Since 2015, it has been the primary source of entertainment for millions of Cubans, as Internet in Cuba has been suppressed for many years with only about a 38.8% Internet penetration rate as of 2018. El Paquete Semanal has its own page that is running in the United States, where one could view its contents and is consistently updated every week.

OpenDoc is a multi-platform software componentry framework standard created by Apple for compound documents, intended as an alternative to Microsoft's Object Linking and Embedding (OLE). As part of the AIM alliance between Apple, IBM, and Motorola, OpenDoc is one of Apple's earliest experiments with open standards and collaborative development methods with other companies—effectively starting an industry consortium. Active development was discontinued in March 1997.

The core idea of OpenDoc is to create small, reusable components, responsible for a specific task, such as text editing, bitmap editing, or browsing an FTP server. OpenDoc provides a framework in which these components can run together, and a document format for storing the data created by each component. These documents can then be opened on other machines, where the OpenDoc frameworks substitute suitable components for each part, even if they are from different vendors. In this way users can "build up" their documents from parts. Since there is no main application and the only visible interface is the document itself, the system is known as document centered.

At its inception, it was envisioned that OpenDoc would allow smaller, third-party developers to enter the then-competitive office software market, able to build one good editor instead of having to provide a complete suite.

The Thing, also known as the Great Seal bug, was one of the first covert listening devices (or "bugs") to use passive techniques to transmit an audio signal. It was concealed inside a gift given by the Soviet Union to W. Averell Harriman, the United States Ambassador to the Soviet Union, on August 4, 1945. Because it was passive, needing electromagnetic energy from an outside source to become energized and activate, it is considered a predecessor of Radio-Frequency Identification (RFID) technology.

The IBM 7030, also known as Stretch, was IBM's first transistorized supercomputer. It was the fastest computer in the world from 1961 until the first CDC 6600 became operational in 1964.

Originally designed to meet a requirement formulated by Edward Teller at Lawrence Livermore National Laboratory, the first example was delivered to Los Alamos National Laboratory in 1961, and a second customized version, the IBM 7950 Harvest, to the National Security Agency in 1962. The Stretch at the Atomic Weapons Research Establishment at Aldermaston, England was heavily used by researchers there and at AERE Harwell, but only after the development of the S2 Fortran Compiler which was the first to add dynamic arrays, and which was later ported to the Ferranti Atlas of Atlas Computer Laboratory at Chilton.

Since the 7030 was much slower than expected and failed to meet its aggressive performance estimates, IBM was forced to drop its price from $13.5 million to only $7.78 million and withdrew the 7030 from sales to customers beyond those having already negotiated contracts. PC World magazine named Stretch one of the biggest project management failures in IT history.

Within IBM, being eclipsed by the smaller Control Data Corporation seemed hard to accept. The project lead, Stephen W. Dunwell, was initially blackballed for his role in the "failure", but as the success of the IBM System/360 became obvious, he was given an official apology and, in 1966 was made an IBM Fellow.

In spite of Stretch's failure to meet its own performance goals, it served as the basis for many of the design features of the successful IBM System/360, which shipped in 1964.

Gematria (; Hebrew: גמטריא or gimatria גימטריה, plural גמטראות or גימטריאות, gimatriot) is the practice of assigning a numerical value to a name, word or phrase by reading it as a number, or sometimes by using an alphanumerical cipher. The letters of the alphabets involved have standard numerical values, but a word can yield several values if a cipher is used.

According to Aristotle (384–322 BCE), isopsephy, based on the Milesian numbering of the Greek alphabet developed in the Greek city of Miletus, was part of the Pythagorean tradition, which originated in the 6th century BCE. The first evidence of use of Hebrew letters as numbers dates to 78 BCE; gematria is still used in Jewish culture. Similar systems have been used in other languages and cultures, derived from or inspired by either Greek isopsephy or Hebrew gematria, and include Arabic abjad numerals and English gematria.

The most common form of Hebrew gematria is used in the Talmud and Midrash, and elaborately by many post-Talmudic commentators. It involves reading words and sentences as numbers, assigning numerical instead of phonetic value to each letter of the Hebrew alphabet. When read as numbers, they can be compared and contrasted with other words or phrases – cf. the Hebrew proverb נכנס יין יצא סוד (nichnas yayin yatza sod, lit. 'wine entered, secret went out', i.e. "in vino veritas"). The gematric value of יין ('wine') is 70 (י=10; י=10; ן=50) and this is also the gematric value of סוד ('secret', ס=60; ו=6; ד=4)‎.

Although a type of gematria system ('Aru') was employed by the ancient Babylonian culture, their writing script was logographic, and the numerical assignments they made were to whole words. Aru was very different from the Milesian systems used by Greek and Hebrew cultures, which used alphabetic writing scripts. The value of words with Aru were assigned in an entirely arbitrary manner and correspondences were made through tables, and so cannot be considered a true form of gematria.

Gematria sums can involve single words, or a string of lengthy calculations. A short example of Hebrew numerology that uses gematria is the word חי (chai, lit. 'alive'), which is composed of two letters that (using the assignments in the mispar gadol table shown below) add up to 18. This has made 18 a "lucky number" among the Jewish people. Donations of money in multiples of 18 are very popular.

In early Jewish sources, the term can also refer to other forms of calculation or letter manipulation, for example atbash.