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Denominator

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Denominator[expr]
    gives the denominator of expr regarded as a rational expression.
    Collects factors of expr that carry a superficially negative
    exponent, inverted; returns 1 when no such factors exist.

Examples

All examples below are verified against the current Mathilda build.

In[1]:= Denominator[(x-1)(x-2)/(x-3)^2]
Out[1]= (-3 + x)^2

In[2]:= Denominator[3/7 + I/11]
Out[2]= 77

Implementation notes

builtin_denominator calls extract_num_den and returns the denominator part (freeing the numerator). extract_num_den recognises Rational[n, d] (returns d); Complex (clears to a common integer denominator); Power[b, e]/Exp[e] with a superficially-negative exponent or a Plus exponent split into positive/negative pieces (the negative-exponent base becomes the denominator); and Times, which recurses into each factor and multiplies the collected denominators. A factor with no denominator contributes 1. Numerator in the same file is the symmetric accessor.

  • Protected, Listable.
  • Picks out terms which have superficially negative exponents.
  • Can be used on rational and complex numbers.

Attributes: Listable, Protected.

Implementation status

Stable — documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

References

Notes & additional examples

Worked examples

In[1]:= Denominator[6/8]
Out[1]= 4

In[1]:= Denominator[(x+1)/(x-1)]
Out[1]= -1 + x

In[1]:= Denominator[a/b + c/d]
Out[1]= 1

In[1]:= Denominator[Together[a/b + c/d]]
Out[1]= b d

In[1]:= Denominator[(x^2-1)/((x-2)^3 (x+5))]
Out[1]= (5 + x) (-2 + x)^3

In[1]:= Denominator[x^(-2) y^3 z^(-1)]
Out[1]= x^2 z

Notes

Denominator returns the bottom of the structural rational form. Rational constants are reduced first, so Denominator[6/8] = 4. A non-fractional expression such as an integer has denominator 1. For symbolic quotients it returns the literal denominator, e.g. -1 + x for (x+1)/(x-1). Because Mathilda does not implicitly Together a sum, Denominator[a/b + c/d] returns 1 (the expression is a Plus with no overall denominator); call Together first to obtain b d.