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Theory of computer sciences
In the formal language theory of computer science, left recursion is a special case of recursion where a string is recognized as part of a language by
Left_recursion
Type of grammar for describing formal languages
parsing, left recursion must be eliminated. Direct recursion, be that left or right, is important in context-free grammars, because there recursion is the
Parsing_expression_grammar
Top-down parser that parses input from left to right
method, see removing left recursion. A simple example for left recursion removal: The following production rule has left recursion on E E -> E '+' T E
LL_parser
Parsing technique
by Frost, Hafiz, and Callaghan, which do accommodate ambiguity and left recursion in polynomial time and which generate polynomial-sized representations
Top-down_parsing
Type of parser
{\displaystyle {\texttt {'aaaa'}}} . Left recursion happens when a grammar production refers to itself as its left-most element, either directly or indirectly
Packrat_parser
Use of functions that call themselves
recursion is a method of solving a computational problem where the solution depends on solutions to smaller instances of the same problem. Recursion solves
Recursion_(computer_science)
Top-down parser utilizing recursion
contain left recursion. Any context-free grammar can be transformed into an equivalent grammar that has no left recursion, but removal of left recursion does
Recursive_descent_parser
Type of a context-free grammar
may or may not be LALR(1). LL grammars cannot have rules containing left recursion. Each LL(k) grammar that is ε-free can be transformed into an equivalent
LL_grammar
Higher-order function that combines several parsers
language Haskell that solve the long-standing problem of accommodating left recursion, and work as a complete top-down parsing tool in polynomial time and
Parser_combinator
Software programming optimization technique
contextual comparison is the key to accommodate indirect (or hidden) left-recursion. When performing a successful lookup in a memotable, instead of returning
Memoization
Process of repeating items in a self-similar way
Recursion occurs when the definition of a concept or process depends on a simpler or previous version of itself. Recursion is used in a variety of disciplines
Recursion
Subroutine call performed as final action of a procedure
tail recursive, which is a special case of direct recursion. Tail recursion (or tail-end recursion) is particularly useful, and is often easy to optimize
Tail_call
Analysing a string of symbols, according to the rules of a formal grammar
implementations of top-down parsing cannot accommodate direct and indirect left-recursion and may require exponential time and space complexity while parsing
Parsing
Computer science and linguistics concept relating to non-terminal production
Maynooth, Co. Kildare, Ireland. Moore, Robert C. (2000), "Removing Left Recursion from Context-free Grammars", Proceedings of the 1st North American Chapter
Recursive_grammar
Parser generator program
SourceForge.[citation needed] ANTLR 4 deals with direct left recursion correctly, but not with left recursion in general, i.e., grammar rules x that refer to
ANTLR
grammar::peg Packrat Tcl Mixed All Free, BSD Grako Packrat + Cut + Left Recursion Python, C++ (beta) Separate All Free, BSD IronMeta Packrat C# Mixed
Comparison of parser generators
Comparison_of_parser_generators
The Panjer recursion is an algorithm to compute the probability distribution approximation of a compound random variable S = ∑ i = 1 N X i {\displaystyle
Panjer_recursion
Type of binary relation
and recursion on S gives primitive recursion. If we consider the order relation (N, <), we obtain complete induction, and course-of-values recursion. The
Well-founded_relation
Form for context-free grammars
sequence of nonterminal symbols. Observe that the grammar does not have left recursions. Every context-free grammar can be transformed into an equivalent grammar
Greibach_normal_form
Type of software bug
Notice that the function on the left must store in its stack exp number of integers, which will be multiplied when the recursion terminates and the function
Stack_overflow
In mathematics, topological recursion is a recursive definition of invariants of spectral curves. It has applications in enumerative geometry, random
Topological_recursion
Divide and conquer sorting algorithm
( n / p ) ) {\displaystyle {\mathcal {O}}\left(p\log \left(n/p\right)\right)} . The expected recursion depth is O ( log ( ∑ i | S i | ) ) = O ( log
Merge_sort
Recursive algorighm in linear algebra
Levinson recursion or Levinson–Durbin recursion is a procedure in linear algebra to recursively calculate the solution to an equation involving a Toeplitz
Levinson_recursion
In programming languages, the object or class the currently running code belongs to
recursion, and means that these methods can be overridden by derived classes or objects. By contrast, direct named recursion or anonymous recursion of
This_(computer_programming)
Tool for analyzing divide-and-conquer algorithms
p(input x of size n): if n < some constant k: Solve x directly without recursion else: Create a subproblems of x, each having size n/b Call procedure p
Master theorem (analysis of algorithms)
Master_theorem_(analysis_of_algorithms)
Rewriting system and type of formal grammar
above to the earlier recursion, one gets: Axiom First recursion Second recursion Third recursion Fourth recursion Seventh recursion, scaled down ten times
L-system
Algorithm for integer multiplication
products can be computed by recursive calls of the Karatsuba algorithm. The recursion can be applied until the numbers are so small that they can (or must)
Karatsuba_algorithm
Type of algorithm in computer science
corecursion is a type of operation that is dual to (structural) recursion. Whereas recursion consumes a data structure by first handling the topmost layer
Corecursion
Programming language
like other programming languages would overflow its stack attempting left recursion. META II uses a $ (zero or more) sequence operator. The expr parsing
META_II
Pattern defining an infinite sequence of numbers
(analysis of algorithms) Mathematical induction Orthogonal polynomials Recursion Recursion (computer science) Time scale calculus Jacobson, Nathan, Basic Algebra
Recurrence_relation
Alternate way to define a function in APL
separated by ⋄ or new-lines, wherein ⍺ denotes the left argument and ⍵ the right, and ∇ denotes recursion (function self-reference). For example, the function
Direct_function
Fractal composed of triangles
Sierpiński pyramid recursion (8 steps)
Sierpiński_triangle
Generalization of addition, multiplication, exponentiation, tetration, etc.
copies of }}a},\quad n\geq 2} It may also be defined according to the recursion rule part of the definition, as in Knuth's up-arrow version of the Ackermann
Hyperoperation
Proof method in mathematical logic
induction. Structural recursion is a recursion method bearing the same relationship to structural induction as ordinary recursion bears to ordinary mathematical
Structural_induction
Family of higher-order functions
without reference to the recursive case (here, on its left i.e., in its first argument), then the recursion would stop. This means that while foldr recurses
Fold_(higher-order_function)
Divide and conquer sorting algorithm
obvious that when recursing on (lo..p) and (p+1..hi), because the left half of the recursion includes the returned index, it is the partition function's job
Quicksort
Quickly growing function
stack reflects the recursion depth. As the reduction according to the rules {r4, r5, r7} involves a smaller maximum depth of recursion, this computation
Ackermann_function
Function computable with bounded loops
composition h ∘ g 1 {\displaystyle h\circ g_{1}} is obtained. Primitive recursion operator ρ {\displaystyle \rho } : Given the k-ary function g ( x 1 ,
Primitive_recursive_function
Defining elements of a set in terms of other elements in the set
\end{aligned}}} This definition is valid for each natural number n, because the recursion eventually reaches the base case of 0. The definition may also be thought
Recursive_definition
Higher-order function which returns some fixed point of the input function
or a for loop. Used in this way, the Y combinator implements general recursion. The lambda calculus does not have global names, for a function to refer
Fixed-point_combinator
Study of computable functions and Turing degrees
Computability theory, also known as recursion theory, is a branch of mathematical logic, computer science, and the theory of computation that originated
Computability_theory
Programming paradigm based on objects
self, to refer to the current object. In languages that support open recursion, a method in an object can call other methods in the same object, including
Object-oriented_programming
Class of algorithms
approach (left) as well as iterative approach (right). Implementations in iterative approach are able to avoid the drawbacks of recursion, particularly
Tree_traversal
American theoretical physicist
Britto on a method calculating scattering amplitudes known as the BCFW recursion relations. Witten has been honored with numerous awards including a MacArthur
Edward_Witten
programming formalisms — such as restricted formal languages, type systems, or recursion schemes — whose expressive power coincides exactly with a given complexity
Implicit computational complexity
Implicit_computational_complexity
Limited form of tree data structure
k-ary tree) Merkle tree Optimal binary search tree Random binary tree Recursion (computer science) Red–black tree Rope (data structure) Self-balancing
Binary_tree
Concept in theoretical computer science
for electrical engineers and technical specialists. Discusses recursion, partial-recursion with reference to Turing Machines, halting problem. A reference
Busy_beaver
Logic puzzle forming a picture in a grid
the next step of the solution even without contradictions and deeper recursion. However, finding such sets is usually as difficult as finding contradictions
Nonogram
Non-comparative lexicographical sorting algorithm
required by LSD. However, MSD sorts are more amenable to subdivision and recursion. Each bucket created by an MSD step can itself be radix sorted using the
Radix_sort
Web browser developed by Google
performed poorly on recursion-intensive benchmarks, such as those of Google, because the Mozilla team had not implemented recursion-tracing yet. Two weeks
Google_Chrome
One of several equivalent definitions of a computable function
{\displaystyle a+b=R^{2}\left(U_{1}^{1},;S_{1}^{3}(S,U_{2}^{3})\right)} Fibonacci number McCarthy 91 function Recursion theory Recursion Recursion (computer science)
General_recursive_function
Size of a mathematical ball
{d}{dR}}V_{n}(R)={\frac {n}{R}}V_{n}(R).} Thus, An − 1(R) inherits formulas and recursion relationships from Vn(R), such as A n − 1 ( R ) = 2 π n / 2 Γ ( n 2 )
Volume_of_an_n-ball
Coefficients coupled with angular momentum
factor is always an integer, so it remains the same when transposed to the left, and the inverse relation follows upon making the substitution m3 → −m3:
3-j_symbol
Formal system in mathematical logic
such as products, coproducts or natural numbers (System T) or even full recursion (like PCF). In contrast, systems that introduce polymorphic types (like
Simply_typed_lambda_calculus
Binary tree variant
allowing tree traversal without recursion and the extra storage (proportional to the tree's depth) that recursion requires. "A binary tree is threaded
Threaded_binary_tree
Type of transfinite numbers
epsilon zero), which can be viewed as the "limit" obtained by transfinite recursion from a sequence of smaller limit ordinals: ε 0 = ω ω ω ⋅ ⋅ ⋅ = sup { ω
Epsilon_number
Algorithm for finding max graph matchings
graphs and directly addresses these challenges. It first introduces a new recursion-free Blossom algorithm. In the traditional algorithm, a blossom is contracted
Blossom_algorithm
American mathematician and logician (1897 – 1954)
Post had been interested in astronomy, but at the age of twelve lost his left arm in a car accident. This loss was a significant obstacle to being a professional
Emil_Leon_Post
General-purpose programming language
FORTRAN 77, many F77 compilers supported recursion as an option, and the Burroughs mainframes, designed with recursion built-in, did so by default. It became
Fortran
Finite or infinite ordered list of elements
using recursion. This is in contrast to the definition of sequences of elements as functions of their positions. To define a sequence by recursion, one
Sequence
Type of geomantic divination
interpreting a series of figures formed by a randomized process that involves recursion, followed by analyzing them, often augmented with astrological interpretations
Arabic_geomancy
Computation model defining an abstract machine
Kleene and J. B. Rosser by use of Church's lambda-calculus and Gödel's recursion theory (1934). Church's paper (published 15 April 1936) showed that the
Turing_machine
System to identify resources on a network
to excessive length. RD: 1 bit Recursion Desired, indicates if the client means a recursive query. RA: 1 bit Recursion Available, in a response, indicates
Domain_Name_System
1957 technique for modelling problems of decision making under uncertainty
forward recursion or backward recursion algorithms, as outlined below. Stochastic dynamic programs can be solved to optimality by using backward recursion or
Stochastic dynamic programming
Stochastic_dynamic_programming
Operations on ordinals that extend classical arithmetic
set that represents the result of the operation or by using transfinite recursion. In addition to these standard operations for ordinals, there are also
Ordinal_arithmetic
Mathematical function characterizing set membership
function. (See paragraph below about the use of the inverse in classical recursion theory.) Given a probability space (Ω,ℱ,P) with A∈ℱ, the indicator random
Indicator_function
Coefficients in angular momentum eigenstates of quantum systems
space inner product. From the formal definition of angular momentum, recursion relations for the Clebsch–Gordan coefficients can be found. There also
Clebsch–Gordan_coefficients
spreadsheet are usually called "rows". recursion Occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines
Glossary_of_computer_science
Sequence of operations for a task
Undecidable, p. 237ff. Kleene's definition of "general recursion" (known now as mu-recursion) was used by Church in his 1935 paper An Unsolvable Problem
Algorithm
Representation theory
j(p)-j(q)=\left({1 \over p}-{1 \over q}\right)\prod _{n,m=1}^{\infty }(1-p^{n}q^{m})^{c_{nm}}} for the elliptic modular function j. Peterson gave a recursion formula
Weyl_character_formula
Rule system for formal languages
become enclosed by matching parentheses; and the third rule terminates the recursion. A second canonical example is two different kinds of matching nested
Context-free_grammar
Product of numbers from 1 to n
by a constant factor at each level of recursion, so the total time for these steps at all levels of recursion adds in a geometric series to O ( n log
Factorial
Polynomial sequence
_{5}(x)&=\left(2{\sqrt {15}}\,\pi ^{\frac {1}{4}}\right)^{-1}\,\left(4x^{5}-20x^{3}+15x\right)\,e^{-{\frac {1}{2}}x^{2}}.\end{aligned}}} Following recursion relations
Hermite_polynomials
Method in numerical analysis
_{1}(x)+a_{2}T_{2}(x)+\cdots +a_{n}T_{n}(x).} The coefficients in the recursion relation for the Chebyshev polynomials are α ( x ) = 2 x , β = − 1 , {\displaystyle
Clenshaw_algorithm
Algorithm to compare text strings using wildcard syntax
characters left to match.) Git/Rsync's wildmatch ABORT also covers invalid inputs. The new INN uwildmat does the same. Asterisk advancement in recursion. This
Matching_wildcards
Muran language
researchers, numerals and counting are based on recursion in the language, the absence of recursion in their language entails a lack of counting. That
Pirahã_language
Mathematical algorithm
{tr} A^{k}~;...} Observe A−1 = − Mn /c0 = (−1)n−1Mn/detA terminates the recursion at λ. This could be used to obtain the inverse or the determinant of A
Faddeev–LeVerrier_algorithm
Arithmetic operation
application of the recursion theorem on the partially ordered set ℕ2. On the other hand, some sources prefer to use a restricted recursion theorem that applies
Addition
Relation between deterministic and nondeterministic space complexity
call halves the parameter k {\displaystyle k} , the number of levels of recursion is ⌈ log 2 n ⌉ {\displaystyle \lceil \log _{2}n\rceil } . Each level
Savitch's_theorem
{{cite web}}: Missing or empty |url= (help) Explanation on StackOverflow: [1], [2], [3] Blogs: [4] Talks: [5] Recursion schemes Haskell package v t e
Paramorphism
Israeli military operation against Syria
from the original on 5 November 2025. Retrieved 7 January 2026. "New recursion: Israeli forces infiltrate Khan Arnaba and arrest four members of a family"
Israeli invasion of Syria (2024–present)
Israeli_invasion_of_Syria_(2024–present)
Mathematical proposition equivalent to the axiom of choice
directly using transfinite recursion, still assuming the axiom of choice. For that, see for example Transfinite recursion theorem § Example: a basis construction
Zorn's_lemma
Term that does not contain any variables
contains no variables. Ground terms may be defined by logical recursion (formula-recursion): Elements of C {\displaystyle C} are ground terms; If f ∈ F
Ground_expression
Algorithm for the kth smallest element in an array
eliminating the tail recursion with a loop: function select(list, left, right, k) is loop if left = right then return list[left] pivotIndex := ... //
Quickselect
Representation of data of various types in lambda calculus
The Y combinator may be used to implement the recursion. Create a new function called div by; In the left hand side divide1 → div c {\displaystyle
Church_encoding
Fractal named after mathematician Benoit Mandelbrot
connectedness locus has also been studied; here one considers the two-parameter recursion z ↦ z 3 + 3 k z + c {\displaystyle z\mapsto z^{3}+3kz+c} , whose two critical
Mandelbrot_set
Programming function applied recursively to its previous result
-> Either a (b, a, b)) -> b -> Tree a ana unspool x = case unspool x of Left a -> Leaf a Right (l, v, r) -> Branch (ana unspool l) v (ana unspool r) To
Anamorphism
Recursively defined sequence of continuous closed plane fractal curves
an even number of times and you halve the length of the line at each recursion. If you recur to an odd depth (order is odd) then you end up turned 60
Sierpiński_curve
Abstract machine used in a formal logic and theoretical computer science
function) Successor function Identity function Composition function Primitive recursion (induction) μ operator (unbounded search operator) The authors show that
Counter_machine
Functions whose execution can be paused
coroutines for state machines or concurrency is similar to using mutual recursion with tail calls, as in both cases the control changes to a different one
Coroutine
Functions of an angle
previously, and conversely one can obtain these series from elementary recursion relations derived from the differential equation. Being defined as fractions
Trigonometric_functions
Type of matrix factorization
is −1. Matrix formulae to calculate rows and columns of LU factors by recursion are given in the remaining part of Banachiewicz's paper as Eq. (2.3) and
LU_decomposition
System of complete and orthogonal polynomials
coefficients of powers of t in the resulting expansion gives Bonnet's recursion formula ( n + 1 ) P n + 1 ( x ) = ( 2 n + 1 ) x P n ( x ) − n P n − 1
Legendre_polynomials
Linked node hierarchical data structure
each child can be treated like the root node of its own subtree, making recursion a useful technique for tree traversal. In contrast to linear data structures
Tree_(abstract_data_type)
Algorithm that arranges lists in order
sorted; sometimes O(log n) additional memory is considered "in-place". Recursion: Some algorithms are either typically recursive or typically non-recursive
Sorting_algorithm
Sierpiński–Menger type of fractal
1/3-scaled corner cubes and the central one from each cube left from the previous recursion (lighter) or by removing only corner cubes (heavier). In one
Mosely_snowflake
referencing The Princess Diaries.[citation needed] "recursion( see it )" shows a "Did you mean: recursion", linking to the same search over and over again
List_of_Google_Easter_eggs
Mathematical logic concept
complexity class containing all computably enumerable sets is RE. In recursion theory, the lattice of c.e. sets under inclusion is denoted E {\displaystyle
Computably_enumerable_set
General-purpose programming language
Retrieved 26 January 2026. van Rossum, Guido (22 April 2009). "Tail Recursion Elimination". Neopythonic.blogspot.be. Archived from the original on 19
Python_(programming_language)
Term in probability theory
probability mass function satisfies the recurrence formula (the Panjer recursion) p k p k − 1 = a + b k , k = 1 , 2 , 3 , … {\displaystyle {\frac
(a,b,0) class of distributions
(a,b,0)_class_of_distributions
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LEFT RECURSION
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