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  2. Euler's totient function - Wikipedia

    en.wikipedia.org/wiki/Euler's_totient_function

    In other words, it is the number of integers k in the range 1 ≤ k ≤ n for which the greatest common divisor gcd(n, k) is equal to 1. [2] [3] The integers k of this form are sometimes referred to as totatives of n. For example, the totatives of n = 9 are the six numbers 1, 2, 4, 5, 7 and 8.

  3. Privacy-enhancing technologies - Wikipedia

    en.wikipedia.org/wiki/Privacy-enhancing_technologies

    [8] In 2003, Borking, Blarkom and Olk reviewed the technologies from a data protection perspective in their Handbook of privacy enhancing technologies. [1] In 2007, Fritsch published an historic, taxonomic and practical overview of contemporary privacy-enhancing technology for the Internet for the research project PETWeb. [9]

  4. Protected health information - Wikipedia

    en.wikipedia.org/wiki/Protected_health_information

    Names; All geographical identifiers smaller than a state, except for the initial three digits of a zip code if, according to the current publicly available data from the U.S. Bureau of the Census: the geographic unit formed by combining all zip codes with the same three initial digits contains more than 20,000 people; the initial three digits of a zip code for all such geographic units ...

  5. List of formulae involving π - Wikipedia

    en.wikipedia.org/wiki/List_of_formulae_involving_π

    where C is the circumference of a circle, d is the diameter, and r is the radius.More generally, = where L and w are, respectively, the perimeter and the width of any curve of constant width.

  6. Publisher Item Identifier - Wikipedia

    en.wikipedia.org/wiki/Publisher_Item_Identifier

    The Publisher Item Identifier (PII) is a unique identifier used by a number of scientific journal publishers to identify documents. [1] It uses the pre-existing ISSN or ISBN of the publication in question, and adds a character for source publication type, an item number, and a check digit.

  7. Pi - Wikipedia

    en.wikipedia.org/wiki/Pi

    The number π (/ p aɪ / ⓘ; spelled out as "pi") is a mathematical constant, approximately equal to 3.14159, that is the ratio of a circle's circumference to its diameter.It appears in many formulae across mathematics and physics, and some of these formulae are commonly used for defining π, to avoid relying on the definition of the length of a curve.

  8. Golden ratio - Wikipedia

    en.wikipedia.org/wiki/Golden_ratio

    The golden ratio φ and its negative reciprocal −φ −1 are the two roots of the quadratic polynomial x 2 − x − 1. The golden ratio's negative −φ and reciprocal φ −1 are the two roots of the quadratic polynomial x 2 + x − 1. The golden ratio is also an algebraic number and even an algebraic integer.

  9. Phi coefficient - Wikipedia

    en.wikipedia.org/wiki/Phi_coefficient

    In statistics, the phi coefficient (or mean square contingency coefficient and denoted by φ or r φ) is a measure of association for two binary variables.. In machine learning, it is known as the Matthews correlation coefficient (MCC) and used as a measure of the quality of binary (two-class) classifications, introduced by biochemist Brian W. Matthews in 1975.