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A Question to Trouble Even the Best of You

  1. Apr 5, 2007 #1
    Find the smallest possible arithmetic sequence consisting of seven primes.

    For example, the smallest possible arithmetic sequence consisting of five primes is: 5, 11, 17, 23, 29.

    By the way, by "smallest possible" I mean that the last term in the sequence must be the smallest possible of all such sequences.
  2. jcsd
  3. Apr 6, 2007 #2

    Gib Z

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    There is no such sequence.
  4. Apr 6, 2007 #3
    Gib Z, you might want to think about that statement some more. It has been proven that there are arithmetic sequences of primes of any finite length.
  5. Apr 7, 2007 #4

    Gib Z

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    Shhh! lol

    Ill reword it then, There is no such sequence that I can be bothered to find and that has any mathematical value. To find such a sequence all one needs is some programming knowledge and to be really bored.
  6. Apr 7, 2007 #5


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    Dearly Missed

    What can readily be established is that any member of such a sequence must have the same last digit.

    So there, I have narrowed it down immensely. :smile:
  7. Apr 7, 2007 #6
    Yeah, is this a computing problem or a math problem?

    (Actually, it sounds more like a Google problem)
  8. Apr 7, 2007 #7

    Gib Z

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    lol adityab88 you google recruit!!!!
  9. Apr 7, 2007 #8
    What type of programming do we need to solve this sort of mathematical problems? C++ ?
  10. Apr 7, 2007 #9

    Gib Z

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    No idea don't ask me lol.

    If you want, start a thread here inquiring about this. But not C++ I know.
  11. Apr 8, 2007 #10
    Unfortunately, C++ is the only programming language I can use.

    Regarding adityab88's question, here is a solution I've just gotten:
    1, 11, 31, 61, 101, 151, 211, 281
    And it's the smallest possible because 361 is not a prime
  12. Apr 8, 2007 #11
    However this is not an arithmetic sequence because consecutive terms do not differ by one single constant.

    ie. 11-1=10, whereas 31-11=20, 61-31=30 etc..
  13. Apr 8, 2007 #12
    For all terms in the sequence:

    [tex] X_n = X_{n-1} + ( (n-1) * 10 ) [/tex]

    Starting from (n = 1), [tex] X_0 = 1 [/tex]

    Is my solution too bad? :frown:
  14. Apr 8, 2007 #13
    The only thing wrong with it is that it is not an arithmetic sequence.

    An arithmetic sequence has terms of the form: an=a0+n*d

    Where a0 is the first term, and d is the common difference between terms.
  15. Apr 8, 2007 #14
    hello ... ppl
    i am quite sure you do not need programming equipment.
    just your brains will suffice; i am sure of this becuase i saw this question on a maths paper, where computers were not allowed

    this is all i have: the difference between the consecutive primes has to be a multiple of 30, i have shown this using some number theory. so, for example, the difference could be 60; a sequence like this (7, 67, 127, ...).

    good luck and have fun!
  16. Apr 8, 2007 #15
    The spacing also can't be 30:

    7, 37, ..., 187 doesn't work because 187 = 11*17. But for any prime [itex]p \neq 7[/itex], we have [itex] p \equiv 2k[/itex] (mod 7) for some k, [itex]1 \leq k \leq 6[/itex]. Since 30 mod 7 = 2 that means that p+30q is divisible by 7 for some q, 1<q<=6.

    That can be generalized to remove some other spacings too; I'm tired and will figure it out in the morning!

    Edit: OK, I lied about figuring it out later!

    You can also eliminate all other spacings, except those which are products of 7, in cases for any sequence which does not start with 7. So the sequence has to start with 7, or the spacing is a multiple of 7 (ie. 210, 420, etc.).

    A little bit of experimenting then yields the sequence: 7, 157, 307, 457, 607, 757, 907
    Last edited: Apr 8, 2007
  17. Apr 8, 2007 #16

    Gib Z

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    Thats top stuff there Data.
  18. Apr 12, 2007 #17
    good stuff

    good stuff
    how long did it take you to "experiment" and find the final sequence? cause i was looking for it, too, but my experimentation did not really work.
  19. Apr 12, 2007 #18
    About five seconds :tongue:. It makes sense to start at 7; if you can find a sequence starting with 7 that has spacing smaller than 210 (and there's no smaller sequence starting with 7), then that's obviously the smallest one.

    Of course, from what I've done there was no particular reason to confine myself to sequences starting with 7, so it would be fair to consider it a lucky coincidence.
  20. Apr 12, 2007 #19


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    What is an "arithmetical sequence"?
  21. Apr 12, 2007 #20
    An arithmetic sequence is one of the form [itex](n,n+k,n+2k,n+3k,...,n+mk,...)[/itex].
  22. Apr 13, 2007 #21
    To me it makes sense to start with 199 and find "minimum" prime-sequence of 10 terms with spacing 210 :

    199, 409, 619, 829, 1039, 1249, 1459, 1669, 1879, 2089 [/tex].

    But it took me longer time ,about 10 seconds:tongue:
  23. Apr 13, 2007 #22
    I've written a mini-introduction to arithmetic progressions https://www.physicsforums.com/blogs/edgardo-22482/arithmetic-progressions-mini-introduction-887/ [Broken]
    Last edited by a moderator: May 2, 2017
  24. Apr 13, 2007 #23
    what about 31,41,51,61,71,81,91 ?

    are there any non-primes in there?

    but even if that is right (i doubt it is, i'm too tired to see the factors i probably should) i cheated b/c i guessed
  25. Apr 13, 2007 #24
    51 and 81 are both composite.
  26. Apr 14, 2007 #25

    haha 9*9 = 81 yep im tired

    Any idea where i could get a description of the concepts they are talking about? (I don't want to hijack the thread)
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