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The numbers in parentheses apply to the numeral left of themselves, and are not part of that number, but part of a notation of uncertainty. They apply to the least significant digits . For instance, 1.007 94 (7) stands for 1.007 94 ± 0.000 07 , while 1.007 94 (72) stands for 1.007 94 ± 0.000 72 . [ 20 ]
Such a number is algebraic and can be expressed as the sum of a rational number and the square root of a rational number. Constructible number: A number representing a length that can be constructed using a compass and straightedge. Constructible numbers form a subfield of the field of algebraic numbers, and include the quadratic surds.
The digits in the base and exponent (10 3 or 10 −2) are considered exact numbers so for these digits, significant figures are irrelevant. Explicitly state the number of significant figures (the abbreviation s.f. is sometimes used): For example "20 000 to 2 s.f." or "20 000 (2 sf)".
The repeating decimal commonly written as 0.999... represents exactly the same quantity as the number one. Despite having the appearance of representing a smaller number, 0.999... is a symbol for the number 1 in exactly the same way that 0.333... is an equivalent notation for the number represented by the fraction 1 ⁄ 3. [437]
A real number is called computable if there exists an algorithm that yields its digits. Because there are only countably many algorithms, [24] but an uncountable number of reals, almost all real numbers fail to be computable. Moreover, the equality of two computable numbers is an undecidable problem.
This includes both quantification of the form "exactly k objects exist such that …" as well as "infinitely many objects exist such that …" and "only finitely many objects exist such that…". The first of these forms is expressible using ordinary quantifiers, but the latter two cannot be expressed in ordinary first-order logic. [4]
A list of articles about numbers (not about numerals). Topics include powers of ten, notable integers, prime and cardinal numbers, and the myriad system.
In mathematics, the notion of number has been extended over the centuries to include zero (0), [3] negative numbers, [4] rational numbers such as one half (), real numbers such as the square root of 2 and π, [5] and complex numbers [6] which extend the real numbers with a square root of −1 (and its combinations with real numbers by adding or ...