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Apart

Status: Stable

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

Description

Apart[expr] rewrites a rational expression as a sum of terms with minimal denominators.
Apart[expr, var] treats all variables other than var as constants.
Option Extension -> alpha factors the denominator over Q(alpha) before
decomposition, splitting (x^2 - 2) into (x - Sqrt[2])(x + Sqrt[2]) under
Extension -> Sqrt[2] and producing the corresponding linear-factor
partial fractions.  Default Extension -> None decomposes over Q.

Examples

All examples below are verified against the current Mathilda build.

In[1]:= Apart[1/((1+x)(5+x))]
Out[1]= -1/4/(5 + x) + 1/4/(1 + x)

In[2]:= Apart[(x^5-2)/((1+x+x^2)(2+x)(1-x))]
Out[2]= 2 - x - 34/9/(2 + x) + 1/9/(-1 + x) + (-1 - 1/3 x)/(1 + x + x^2)

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

In[4]:= Apart[1/(y^(2/3) - 1/y^(1/3))]
Out[4]= 1/3/(-1 + y^(1/3)) + (1/3 - 1/3 y^(1/3))/(1 + y^(1/3) + y^(2/3))

In[5]:= Apart[1/(x^2 - 2), x, Extension -> Sqrt[2]]
Out[5]= -1/2 1/(Sqrt[2] (Sqrt[2] + x)) + 1/2 1/(Sqrt[2] (-Sqrt[2] + x))

Implementation notes

Algorithm. builtin_apart strips an optional Extension -> α (with Automatic running extension_autodetect for a single algebraic generator) and dispatches to apart_impl. Inexact coefficients route through internal_rationalize_then_numericalize. The core does an undetermined-coefficients partial-fraction decomposition over Q (or Q(α)):

  1. Thread over List/relational/logical heads when the argument is one.
  2. For a radical generator (fractional rational exponents), substitute u -> g^m, recurse, and back-substitute (poly_find_radical_gen / poly_subst_radical_*).
  3. Combine to a single fraction with Together, pick the partial-fraction variable (explicit 2nd arg, else the lexicographically-last collected variable), and split into numerator N and denominator D (bailing back to the Together'd form if either is not polynomial in the variable, checked by PolynomialQ).
  4. Polynomial-divide N by D (PolynomialQuotient/PolynomialRemainder) to get the polynomial part Q and proper remainder R.
  5. Factor the denominator (over the extension if given), separate constant factors into a scalar C, and for each irreducible base b_i of multiplicity k_i set up unknown numerators of degree deg(b_i)-1 over each power b_i^j.
  6. Build the linear system by expanding each basis-times-var^r term, reading coefficients with get_coeff, and solving via RowReduce; assemble the sum Q + sum A_ij / b_i^j, factoring each solved coefficient.

Data structures. Everything is Expr trees driven through eval_and_free/expr_expand. The linear system is a dense Expr*** augmented matrix (S rows by S+1 columns, S = deg(D)) of coefficient expressions handed to RowReduce; factored denominator bases and their exponents are kept in parallel Expr**/int64* arrays.

Complexity / limits. Dominated by Factor[D] and the O(S^3) RowReduce over symbolic entries. Partial fractions are undefined when N/D are not polynomial in the chosen variable (handled by returning the combined fraction), and the matrix path assumes integer/rational coefficient structure.

  • Protected, Listable.
  • Writes expr as a polynomial in var together with a sum of ratios of polynomials with minimal denominators.
  • If var is not specified, intelligently selects the main polynomial variable natively.
  • Implements exact undetermined coefficients algebraically leveraging row-reduced identity expansions over algebraic inputs avoiding recursive fractional losses natively.
  • When Together[expr] produces a numerator or denominator that is not polynomial in the chosen variable (e.g. fractional-power inputs whose Together'd form is y^(1/3)/(y - 1)), the matrix-of-coefficients algorithm cannot apply; Apart returns the Together form unchanged rather than synthesising a spurious zero.
  • Option Extension -> alpha (Phase 0 of the Integrate plan) factors the denominator over Q(alpha) before partial-fraction decomposition runs, splitting reducible-over-extension factors (e.g. x^2 - 2 into (x - Sqrt[2])(x + Sqrt[2]) under Extension -> Sqrt[2]) and producing the corresponding linear-factor partial fractions. The pre-Together step is also extension-aware so any algebraic-number cancellations in numerator/denominator fire before splitting.

Attributes: Listable, Protected.

Implementation status

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

References

  • Geddes, Czapor & Labahn, "Algorithms for Computer Algebra" (1992), on partial fraction decomposition.
  • von zur Gathen & Gerhard, "Modern Computer Algebra", on partial fractions and the extended Euclidean algorithm.
  • K. O. Geddes, S. R. Czapor and G. Labahn, Algorithms for Computer Algebra (Kluwer, 1992), ch. on partial-fraction decomposition.
  • Source: src/parfrac.c
  • Specification: docs/spec/builtins/algebra.md

Notes & additional examples

Worked examples

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

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

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

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

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

In[1]:= Apart[1/(x^2 - 2), Extension -> Sqrt[2]]
Out[1]= -1/2 1/(Sqrt[2] (Sqrt[2] + x)) + 1/2 1/(Sqrt[2] (-Sqrt[2] + x))

Notes

Apart computes the partial-fraction decomposition with respect to the (sole) variable, the inverse of Together. It factors the denominator and splits the quotient into a sum of terms whose denominators are the factors, including repeated-factor terms: 1/(x^2 (x+1)) decomposes into 1/x^2 - 1/x + 1/(1+x), recovering both the 1/x^2 and the lower-order 1/x contributions. Rational residues appear when the factors are linear over the rationals, as in the -1/2 and 3/2 coefficients for (x+2)/(x^2-1). When the numerator degree meets or exceeds the denominator's, Apart first divides out a polynomial part: (x^3 + 1)/(x^2 - 1) becomes x + 1/(-1 + x). Irreducible quadratic factors over Q are kept intact (the (1 - 3/2 x)/(1 + x^2) term), while Extension -> Sqrt[2] splits x^2 - 2 into the conjugate linear factors x +- Sqrt[2] and produces the corresponding partial fractions over Q(Sqrt[2]).