# Equal+number

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**F-number**— In optics, the f number (sometimes called focal ratio, f ratio, or relative aperture [Smith, Warren Modern Lens Design 2005 McGraw Hill] ) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; …72

**Atomic number**— See also: List of elements by atomic number An explanation of the superscripts and subscripts seen in atomic number notation. In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the… …73

**Negative number**— This thermometer is indicating a slightly negative …74

**perfect number**— noun : an integer (as 6) the sum of whose divisors including 1 but excluding itself is equal to itself 28, which . 1 + 2 + 4 + 7 + 14, is a perfect number compare imperfect number * * * Math. a positive number that is equal to the sum of all… …75

**19 equal temperament**— In music, 19 equal temperament, called 19 TET, 19 EDO, or 19 ET, is the tempered scale derived by dividing the octave into 19 equally large steps. Each step represents a frequency ratio of 21/19, or 63.16 cents. Because 19 is a prime number, one… …76

**perfect number**— Math. a positive number that is equal to the sum of all positive integers that are submultiples of it, as 6, which is equal to the sum of 1, 2, and 3. Cf. abundant number, deficient number. [1350 1400; ME] * * * ▪ mathematics a positive… …77

**Complex number**— A complex number can be visually represented as a pair of numbers forming a vector on a diagram called an Argand diagram, representing the complex plane. Re is the real axis, Im is the imaginary axis, and i is the square root of –1. A complex… …78

**0 (number)**— Zero redirects here. For other uses, see Zero (disambiguation). 0 −1 0 1 2 3 4 5 6 7 8 …79

**Highly composite number**— This article is about numbers having many divisors. For numbers factorized only to powers of 2, 3, 5 and 7 (also named 7 smooth numbers), see Smooth number. A highly composite number (HCN) is a positive integer with more divisors than any… …80

**72 equal temperament**— In music, 72 equal temperament, called twelfth tone, 72 tet, 72 edo, or 72 et, is the tempered scale derived by dividing the octave into twelfth tones, or in other words 72 equally large steps. Each step represents a frequency ratio of 21/72, or… …