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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. 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.

  4. Euler's formula - Wikipedia

    en.wikipedia.org/wiki/Euler's_formula

    r = | z | = √ x 2 + y 2 is the magnitude of z and; φ = arg z = atan2(y, x). φ is the argument of z, i.e., the angle between the x axis and the vector z measured counterclockwise in radians, which is defined up to addition of 2π. Many texts write φ = tan −1 ⁠ y / x ⁠ instead of φ = atan2(y, x), but the first equation needs ...

  5. 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.

  6. 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.

  7. 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.

  8. Proof that π is irrational - Wikipedia

    en.wikipedia.org/wiki/Proof_that_π_is_irrational

    In each case, cannot be , because otherwise it would follow from claim 1 that each + () would be , which would contradict claim 2. Now, take a natural number c {\displaystyle c} such that all three numbers b c / k , {\displaystyle bc/k,} c k / x 2 , {\displaystyle ck/x^{2},} and c / x 2 {\displaystyle c/x^{2}} are integers and consider the sequence

  9. Pi-system - Wikipedia

    en.wikipedia.org/wiki/Pi-system

    In mathematics, a π-system (or pi-system) on a set is a collection of certain subsets of , such that . is non-empty.; If , then .; That is, is a non-empty family of subsets of that is closed under non-empty finite intersections.