Forty-two's fame has nothing to do with any mathematical property distinguishing it from its neighbors — it's a purely cultural phenomenon, and one of the cleanest examples of a number becoming famous through fiction rather than through any provable arithmetic fact.
The reference comes from Douglas Adams's 1979 novel The Hitchhiker's Guide to the Galaxy, in which a vast supercomputer named Deep Thought is built specifically to calculate the answer to "the Ultimate Question of Life, the Universe, and Everything." After seven and a half million years of computation, Deep Thought delivers its answer: 42. The joke, central to the novel, is that the answer is delivered without anyone actually knowing what the original question was — Deep Thought's programmers had never actually formulated the question with enough precision for the numerical answer to be meaningful on its own, a deliberate piece of absurdist commentary on the gap between having an answer and understanding what it actually answers.
Adams himself, when asked directly why he chose 42 specifically rather than any other number, gave characteristically evasive and varying answers over the years, at different points suggesting it was simply a mundane, unremarkable number chosen precisely because it had no inherent significance — which was, in a sense, the entire point of the joke: the "answer to everything" turning out to be an ordinary, forgettable-looking number is itself part of the novel's comedic and philosophical point about the absurdity of seeking a single numerical answer to questions of ultimate meaning.
Despite Adams's own account, the reference has since spawned an enormous amount of retroactive numerological and mathematical searching for hidden significance in 42 — attempts to find special properties in its factorization (42 = 2×3×7, a product of the first three primes after excluding 5, though this specific framing is itself a somewhat cherry-picked way of describing it), or connections to other cultural systems, most of which are best understood as fans and enthusiasts having fun with the reference after the fact rather than anything Adams intended or that reflects a genuine underlying mathematical distinction.
Forty-two has, in the decades since the novel's publication, become one of the most widely recognized numerical pop-culture references in English-language media — used as an Easter egg or deliberate wink across an enormous range of software, films, and other creative works by people who grew up with or later discovered Adams's novel. It's included on this site specifically as a useful contrast case: not every number that shows up on a page like this earned its notability through a genuine, checkable mathematical property (the way 496 or 6,174 did) — some numbers are notable purely as cultural touchstones, and 42 is the cleanest, most self-aware example of that category, given that the joke's entire premise is the mismatch between a specific numerical answer and any real underlying meaning.
The novel's own internal joke has, in a fittingly recursive twist, generated real academic and enthusiast attention over the decades — including semi-serious explorations of 42 in binary (101010, a tidy alternating pattern some fans have pointed to, though this is almost certainly coincidental rather than intended by Adams) and its appearances as an inside-joke Easter egg planted deliberately by programmers and writers across an enormous range of unrelated software and media. It stands as one of the clearest examples in modern popular culture of a number becoming genuinely famous purely through a single, specific fictional reference rather than through any organic cultural or religious tradition, or any distinguishing mathematical property.
Set against its purely fictional fame, 42's actual computed properties are unremarkable: it's an even composite number with eight total divisors (1, 2, 3, 6, 7, 14, 21, 42), not a perfect square, not prime, and not a member of any of the classic special sequences tracked elsewhere on this site. That gap between an unremarkable arithmetic profile and an enormous cultural footprint is, again, exactly the point of the original joke — Adams's deliberate choice of an ordinary-looking number is precisely what makes the fictional "answer" land as absurd rather than mystically significant.
The reference has proven durable enough that "42" is now recognized as a meaningful cultural signal well outside dedicated fandom circles — used in software version numbers, product names, and public installations as a deliberate nod, in a way that few other fictional numerical references from 20th-century literature have achieved with comparable staying power or recognizability across generations of readers who may never have read the original novel.