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An example of an irrational algebraic number is x 0 = ... However, there is a second definition of an irrational number used in constructive mathematics, ...
In mathematics, an irrational number is any real number that is not a rational number, i.e., one that cannot be written as a fraction a / b with a and b integers and b not zero. This is also known as being incommensurable, or without common measure. The irrational numbers are precisely those numbers whose expansion in any given base (decimal ...
Algebraic number: Any number that is the root of a non-zero polynomial with rational coefficients. Transcendental number: Any real or complex number that is not algebraic. Examples include e and π. Trigonometric number: Any number that is the sine or cosine of a rational multiple of π.
Example: Let a and b be nonzero real numbers. Then the subgroup of the real numbers R generated by a is commensurable with the subgroup generated by b if and only if the real numbers a and b are commensurable, in the sense that a/b is rational. Thus the group-theoretic notion of commensurability generalizes the concept for real numbers.
For example, all rational numbers have degree 1, and an algebraic number of degree 2 is a quadratic irrational. The algebraic numbers are dense in the reals . This follows from the fact they contain the rational numbers, which are dense in the reals themselves.
Otherwise, that cut defines a unique irrational number which, loosely speaking, fills the "gap" between A and B. [3] In other words, A contains every rational number less than the cut, and B contains every rational number greater than or equal to the cut. An irrational cut is equated to an irrational number which is in neither set.
This is because the set of rationals, which is countable, is dense in the real numbers. The irrational numbers are also dense in the real numbers, however they are uncountable and have the same cardinality as the reals. The real numbers form a metric space: the distance between x and y is defined as the absolute value |x − y|.
For example, if a right triangle has legs of the length 1 then the length of its hypotenuse is given by the irrational number . π is another irrational number and describes the ratio of a circle's circumference to its diameter. [22] The decimal representation of an irrational number is infinite without repeating decimals. [23]