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Submitted via IRC for Bytram
What Is an "Almost Prime" Number?
When I saw a math paper with the phrase "almost prime" in the title, I thought it sounded pretty funny. It reminded me of the joke about how you can't be a little bit pregnant. On further thought, though, it seems like someone whose pregnancy is 6 weeks along and who hasn't yet noticed a missed period is meaningfully less pregnant that someone rounding the bend at 39 weeks who can balance a dinner plate on their belly. Perhaps "almost prime" could make sense too.
A number is prime if its only factors are 1 and itself. By convention, the number 1 is not considered to be prime, so the primes start 2, 3, 5, 7, 11, and so on. Hence, a prime number has one prime factor. A number with two prime factors, like 4 (where the two factors are both 2) or 6 (2×3) is definitely less prime than a prime number, but it kind of seems more prime than 8 or 30, both of which have three prime factors (2×2×2 and 2×3×5, respectively). The notion of almost primes is a way of quantifying how close a number is to being prime.
(Score: 3, Insightful) by pkrasimirov on Monday November 05 2018, @02:37PM (1 child)
Hmm... methinks if this can be used in encryption somehow... like a crypto that can result in few plaintexts but only one of them makes sense (checksum matches).
(Score: 1, Informative) by Anonymous Coward on Monday November 05 2018, @02:54PM
Sure. 2-almost primes (that is, products of two primes) are an essential part of the RSA algorithm.