Spacing between Prime Numbers
Date: 11/08/2005 at 09:44:25 From: Jared Subject: Separation Between Prime Numbers I'm aware that there is no known pattern as of yet to define the frequency of primes occurring; however, I'm also aware that over fifteen million primes have been identified and checked by reputable mathematics sources. My question is this. Given our knowledge of primes is there a way to determine the specific "distance" between two primes without reading through a general list of known primes? Even if this is not possible (as I suspect), I'd like to know where the first place is that the difference between two primes exceeds two thousand and how to find any subsequent spacings of this magnitude. The fact that no known pattern exists has it made it difficult to determine and locate specific spacings between primes. The only way I've been able to find to determine a set of primes with a specific spacing between them has been to read through various data bases of known primes, which has thus far been fruitless.
Date: 11/08/2005 at 14:50:08 From: Doctor Vogler Subject: Re: Separation Between Prime Numbers Hi Jared, Thanks for writing to Dr. Math. Perhaps you don't realize how much about primes is known. In fact, there is a simple formula that tells us roughly how many primes will be found in a certain range, and you can use this to say what the _average_ spacing will be. It turns out that the average spacing between primes around the number x will be roughly the natural log of x. For more details about this, see Prime Number Theorem http://mathworld.wolfram.com/PrimeNumberTheorem.html or search our archives or the Internet for "prime number theorem." On the other hand, that doesn't tell you what the individual spacings are. It says that they _generally_ get bigger, if slowly, but in fact it is believed (though this has yet to be proven) that there are infinitely many "twin primes" (which means spacings equal to two, i.e. two primes that differ by two). This is called the Twin Primes Conjecture. If it's true, then that means that the spacings get really big and really small, and this kind of behavior tends to defy simple formulas. In fact, my favorite quote from Prime Number http://mathworld.wolfram.com/PrimeNumber.html is: In a 1975 lecture, D. Zagier commented "There are two facts about the distribution of prime numbers of which I hope to convince you so overwhelmingly that they will be permanently engraved in your hearts. The first is that, despite their simple definition and role as the building blocks of the natural numbers, the prime numbers grow like weeds among the natural numbers, seeming to obey no other law than that of chance, and nobody can predict where the next one will sprout. The second fact is even more astonishing, for it states just the opposite: that the prime numbers exhibit stunning regularity, that there are laws governing their behavior, and that they obey these laws with almost military precision" (Havil 2003, p. 171). It is generally not hard to use factorials (or something similar) to find a span of M consecutive integers that are all composite, but these tend to give very large numbers. For example, all of the integers from (M+1)! + 2 to (M+1)! + M + 1 are composite (and this is M numbers). But (M+1)! is a very large number, so this will usually not give the _first_ time there is a spacing of M, and it doesn't guarantee that the two numbers on either end are prime; so it only shows that the spacing is _at least_ M. To find the first time the spacing is bigger than 2000, you have to compute all prime numbers until you find that first spacing, and this can take a lot of time. But we know that the _average_ spacing won't be 2000 until you get as high as about e^2000 (which is a huge number, with 869 digits). Of course, the first spacing of that size will probably show up before the average spacing gets that big, but you should still expect that it won't show up until you've computed more primes than all the computers in the world today could compute in a billion years. By comparison, the number (2002)! + 2 has 5743 digits, so this is also a huge number, and much bigger than e^2000. If you have any questions about this or need more help, please write back and show me what you have been able to do, and I will try to offer further suggestions. - Doctor Vogler, The Math Forum http://mathforum.org/dr.math/
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