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Built-in Mathematica Symbol

Denominator

Denominator[expr]
gives the denominator of expr.

Denominator picks out terms which have superficially negative exponents. Numerator picks out all remaining terms.

An exponent is "superficially negative" if it has a negative number as a factor.

The standard representation of rational expressions as products of powers means that you cannot simply use Part to extract denominators.

Extract denominator of a rational number:

Extract denominator of a rational expression:

Extract denominator of a rational number:

Extract denominator of a rational expression:

Rational numbers:

Rational expressions:

Select terms with syntactically negative exponents:

All exponents syntactically negative:

No syntactically negative exponents:

Denominator automatically threads over lists:

Find denominators over integers modulo m:

Denominators of trigonometric functions:

Numerator gives the terms without negative exponents:

An expression is a quotient of its numerator and denominator:

Use Cancel to cancel common factors between the numerator and the denominator:

Together writes an expression as a fraction and cancels common terms:

Use ExpandDenominator to directly expand all denominators:

Cyclic addition [more info]: