20070507, 01:19  #1 
6895_{10} Posts 
Counterexamples needed
I'm guessing that there is always a twin prime between n^3 and (n+1)^3, assuming n is a positive integer. However, I have no idea how to prove whether this is correct or not, so could someone give a counterexample to disprove my theory? I tried finding counterexamples, but I haven't found one yet.

20070507, 16:21  #2  
∂^{2}ω=0
Sep 2002
República de California
2·5,813 Posts 
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20070507, 23:14  #3 
2×1,783 Posts 
I did a manual search first. I used a calculator to calculate the cube of two consecutive numbers (10^3 and 11^3, for example), and then checked
http://primes.utm.edu/lists/small/1ktwins.txt to see if there were any twins between those two numbers. I searched until 25^3, and I still couldn't find any counterexamples. So, I wrote a program that searched for counterexamples by checking whether there were any twins between n^3 and (n+1)^3. I stopped the program at n=1500 without finding any counterexamples, because it seemed that the higher n was, the more twins there were between n^3 and (n+1)^3. 
20070507, 23:15  #4 
Aug 2005
Brazil
2×181 Posts 
I've checked it to n=30000 using a (very) crude algorithm.

20070508, 19:20  #5 
∂^{2}ω=0
Sep 2002
República de California
2×5,813 Posts 
In fact, to show just how far number theorists are from proving such a thing (assuming for the purposes of argument that it were true):
1) Chebyshev's theorem (formerly known in a slightly different form as Bertrand's postulate) states that for every n > 1 there is always at least one prime p such that n < p < 2n. As marvelous as Chebyshev's proof (and later ones by the likes of Ramanujan and Erdös) of this fact is, it is an incredibly weak result, especially since the data indicate that the maximum gap between consecutive primes at large n seems to behave as O((log n)^{2}), which is asymptotically miniscule compared to the O(n) maximal gap that is proven by the above theorem. (The formal statement of the alleged logsquared behavior is known Cramér's conjecture; note also that this statement about *relative* gap size is not inconsistent with the proven result that a prime gap may have arbitrarily large *absolute* size) 2) There are some wellknown results about the occurrence of multiple primes in intervals  from the above Wikilink: "Erdös proved that for any positive integer k, there is a natural number N such that for all n > N, there are at least k primes between n and 2n. Erdös also proved there always exist at least two prime numbers p with n < p < 2n for all n > 6. Moreover, one of them is congruent to 1 modulo 4, and another one is congruent to 1 modulo 4. However, given that from the Prime Number Theorem (PNT) we expect on average O(n/log(n)) primes between n and 2n for large n, this is still a comparatively weak proven lower bound. And it's even further from saying anything about the occurrence of *twin* primes in such an interval (except in the sense that twin primes would of course satisfy the +1 modulo 4 requirement), which is moreover asymptotically much larger than your n^3 to n^3+O(n^{2}) conjectured interval. 3) Again quoting from the above page, here is a conjecture in a similar form as yours, but much, much weaker in that it only asks if there is *one* prime in some interval smaller than O(n)  and it's of course unproven: "A similar and still unsolved Legendre's conjecture asks whether for every n > 1, there is a prime p, such that n^{2} < p < (n + 1)^{2}. Again we expect from the PNT that there will be not just one but many primes between n^{2} and (n + 1)^{2}, but in this case the error estimates on the PNT are not (indeed, cannot be) sufficient to prove the existence of even one prime in this interval." 4) Even though it is widely believed to be true, despite centuries of effort by some of the brightest mathematical minds in history, it has not yet been proven that infinitely many twin prime pairs *exist*, much less subject to additional contsraints on their distributional density. So, in short, we are quite a long way from proving anything along the lines you suggest. Good luck! 
20070509, 16:52  #6  
Bronze Medalist
Jan 2004
Mumbai,India
100000000100_{2} Posts 
Excellent reply!
Quote:
Thanks Ernst! That was an excellent up to date reply indeed. I often re read Paul Hoffman's book 'Archimedes' Revenge' [pub.year 1990] for the elaborate section he gives on primes. I quote a para which may be of some relevance for this thread. " A mammoth treatise could be given about what is known and not known about primes. .... It has been proved that there is at least one prime between any number greater then one and its double. ( A surprising consequence of this proof is that there are at least three primes having exactly n digits where n is any positive integer whatever.) [Get a load of that; its not as simple as it sounds] [I have not come across that in a cursory scan of your post though I may have overlooked it in which case I am sorry] To continue [ from the book] what you have already stated " No one knows whether a prime lies between the square of any number greater than one and the square of its successor" Mally Last fiddled with by mfgoode on 20070509 at 16:58 Reason: Adding bracket 

20070509, 19:30  #7 
Einyen
Dec 2003
Denmark
17×181 Posts 
Looking at the number of twin primes between n^3 and (n+1)^3 up to n=5000:
twintest.txt The number is raising all the time, it seems unlikely there should be 0 in any interval above n=5000, but thats far from a proof. 
20070510, 10:56  #8  
Nov 2003
2^{2}×5×373 Posts 
Quote:
HUH?? No! No! No! There is always a prime between n^3 and (n+1)^3. A proof exists. This has nothing to do with the twin prime conjecture. See, for example, Richard Guy's book "Unsolved Problems in NUmber Theory" Let M = n^3. Then (n+1)^3 = n ^3 + 3n^2 + 3n + 1 = M+ O(M^2/3). But a proof exists that there is always a prime between M and M^(11/20 + e) for any e > 0. I also think that the exponent 11/20 may have been lowered slightly. Note that 3M^2/3 is greater than M^(11/20 + e). The Riemann Hypothesis would allow us to have M^(1/2 + e), but no proof yet exists that allows the exponent to be lowered to 1/2. 

20070510, 12:20  #9 
Feb 2006
Denmark
2×5×23 Posts 

20070510, 12:42  #10  
Nov 2003
2^{2}×5×373 Posts 
Quote:
I think that this question does not even merit the word "conjecture" because the evidence is too scant. I would call it an "open question". I don't even know of any work on the expected gap between successive twin primes. They are much rarer than primes themselves. We don't even know if there is always a twin prime between (sufficiently large) n and 2n. 

20070525, 17:18  #11 
Feb 2005
374_{8} Posts 
I have verified the statement for n up to 3,300,000.
No counterexamples exist in this range. 
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