Rationalize[x] converts an approximate number x to a nearby rational with small denominator. Rationalize[x, dx] yields the rational number with smallest denominator that lies ...

Rational is the head used for rational numbers.

Rationals represents the domain of rational numbers, as in x \[Element] Rationals.

Ratios[list] gives the successive ratios of elements in list. Ratios[list, n] gives the n\[Null]^th iterated ratios of list. Ratios[list, {n_1, n_2, ...}] gives the ...

RawBoxes[boxes] is a low-level construct which is formatted as boxes without further interpretation.

RawData[data] is a low-level representation of the contents of a cell in which Show Cell Expression has been toggled.

RayleighDistribution[\[Sigma]] represents the Rayleigh distribution with scale parameter \[Sigma].

ReadList["file"] reads all the remaining expressions in a file and returns a list of them. ReadList["file", type] reads objects of the specified type from a file, until the ...

Read[stream] reads one expression from an input stream and returns the expression. Read[stream, type] reads one object of the specified type. Read[stream, {type_1, type_2, ...

ReadProtected is an attribute that prevents values associated with a symbol from being seen.