Quine is a nonempty [program](program.md) that upon execution prints its own [source code](source_code.md). It takes no input, just prints out the source code, and that without "[cheating](cheating.md)" such as reading the source from a [file](file.md). A [self-replicating](self_replication.md) program of a sort, quine performs a task similar to that of a robot capable of creating copies of itself (after all we humans are such robots). The name *quine* refers to the philosopher Willard Quine and his [paradox](paradox.md) that exhibits a structure similar to self-replicating programs. Quine belongs to the class of standard/[fun](fun.md)/[interesting](interesting.md) programs with which we like to entertain ourselves and that are used for comparing [programming languages](programming_language.md), it's a cousin of such superstars as [hello world](hello_world.md), [compiler bombs](compiler_bomb.md), [99 bottles of beer](99_bottles.md) or [fizzbuzz](fizzbuzz.md), but out of these quine is of the greatest interest to mathematicians.
From [mathematical](math.md) viewpoint quine is a **fixed point** of a [function](function.md) (not to be confused with [fixed_point arithmetic](fixed_point.md)) represented by the [programming language](programming_language.md). That is once we say the programming language is a function f(x), where *x* is source code and the function's output is the program's output, quine is such *x* that *f(x) = x*. **A quine can be written in any [Turing complete](turing_completeness.md) [language](programming_language.md)**, the proof comes from the *fixed point theorem* (which says a function satisfying certain conditions always have a fixed point).
The difficulty in constructing a quine lies indeed in the **[self reference](self_reference.md)** which makes the solution "run away" as we're chasing it: as we are working on the solution (changing the source code), the goal itself (source code we have to output) keeps changing, and so we cannot just single-line print a string literal containing the source we currently have at hand because the string would have to contain itself, making it [infinite](infinity.md) in length. Challenges mathematically similar by this aspect -- i.e. complicated by self reference -- include for example constructing self matching [regular expressions](regex.md) (but for this to be non-trivial additional constraints must be given: let's say that the regex must be enclosed between `/`s) or creating images that spell out their own [hash](hash.md).
Yet a stronger quine is so called **radiation hardened quine**, a quine that remains quine even after any one character from the program has been deleted (found here in [Ruby](ruby.md): https://github.com/mame/radiation-hardened-quine). A **polyglot quine** is quine which is a valid quine in several languages at once. Other plays on the theme of quine include e.g. a program that produces a bigger program which will again produce yet bigger program etc.
Another extension of a quine is **multiquine** -- this is NOT a polyglot quine! Multiquine is a quine written in some programming language *L0*; under normal circumstances this program behaves like a normal quine, but it has an extra feature: when passed a parameter *N* (e.g. through [CLI](cli.md) flag or through standard input), it will print a program in another language, *LN*, which itself is this multiquine (so it can again be used to get back the program in *L0* and so on). I.e. a multiquine is a quine which can switch between several languages.
In the [Text](plaintext.md) [esoteric programming language](esolang.md) every program is a quine (and so also a radiation hardened one).
