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True/False

If $p$ is a $3$-digit prime, then there always exist $p$ consecutive composite numbers.

How to approach this?

Community
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Hyperbola
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    Hint: forget about "3-digit prime", just prove that for any positive integer $P$ there exist $P$ consecutive composite numbers. – Gerry Myerson Jul 08 '12 at 03:14
  • Green-Tao theorem? – Hyperbola Jul 08 '12 at 03:16
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    @Hyperbola: That theorem says that there are arbitrarily long sequences of primes in arithmetic progression. It is neither related to, nor necessary, to prove this. HINT: $(n+1)!$ is composite whenever $n\gt 1$. Can $(n+1)!+2$ be prime? What about $(n+1)!+3$? – Arturo Magidin Jul 08 '12 at 03:16
  • Both will be always composite. – Hyperbola Jul 08 '12 at 03:22
  • @Hyperbola: And can you take it from there? – Arturo Magidin Jul 08 '12 at 03:23
  • Sure thing @ArturoMagidin – Hyperbola Jul 08 '12 at 03:24
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    @Hyperbola: Consider posting your solution as an answer. People can then check it, and after some time you can even accept it. It will also prevent this question from appearing as "unanswered". – Arturo Magidin Jul 08 '12 at 03:25
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    A more difficult question is whether there exist $p$ consecutive composite numbers preceded and followed by primes. Without the "3 digit" restriction this is not known. With it, it is a matter for explicit computation (see http://www.trnicely.net/gaps/gaplist.html#MainTable) – Robert Israel Jul 08 '12 at 06:38

3 Answers3

4

$P_k =(s + 1)! + k$ for $k = 2$ to $(s + 1)$

are $s$ consecutive positive integers

as $P_k$ is always divisible by $k$, hence the statement is true

Prasanth S
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Bazinga
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Hint $\rm\ 1<a,b,c\:|\:n\:\Rightarrow\:n\!+\!a,\,n\!+\!b,\,n\!+\!c\,$ are composite. Now put $\rm\,a,b,c = 2,3,4\ldots$ and $\rm n =\,$ __

Bill Dubuque
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The statement is false!
The first 3 digit prime is 101, the next prime is 103; difference is 2 not 101! Take another 3 digit prime, 131 the next prime is 137; difference is 6 not 131! You can look up any of the 3 digit primes and by inspection the difference is well under 100. Ron

Ron Hallam
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