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Sequences problem

  1. Feb 13, 2013 #1

    trollcast

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    1. The problem statement, all variables and given/known data
    For an increasing sequence of numbers, how many other sequences could this be the average sequence of.

    2. Relevant equations

    Where the average sequence, a = 0.5( s + s[i+1] )

    3. The attempt at a solution

    If theres n terms in the original sequence.
    The number of differences between consecutive terms is (n - 1)
    Find all these, (n-1) and find the lowest difference.
    Then this lowest difference + 1 is your answer?

    eg.
    s = 1, 3, 6, 10, 12
    s[i+1] - s = 2, 3, 4, 2

    The lowest difference here is 2 so theres 2 possible sequences for which s is the average of?
     
  2. jcsd
  3. Feb 13, 2013 #2

    I like Serena

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    Hey trollcast! :smile:

    Suppose we start with the simplest possible sequence we can think of: 1,2,3,4,...

    Now we'd look for another sequence that has the same averages.
    To get the same average, if we increase a specific number by some amount ε > 0, the next number must be decreased by that same amount ε.
    So we'd add an alternating sequence ##(-1)^i ε##.
    Since the resulting sequence still has to be increasing, that ε must be less than (1/2).

    So we'd start with the sequence specified by ##s_i = i## and we'd end up with the sequence specified by ##s_i' = i + (-1)^i ε## with ##0 < ε < \frac 1 2 ##.

    How many of those ε's are there?
     
  4. Feb 13, 2013 #3

    trollcast

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    Would there be 0 if the sequence has to be integers?
     
  5. Feb 13, 2013 #4

    I like Serena

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    Yes if we're talking about the sequence 1,2,3,...

    But for another sequence like 0,10,20,30,40,...
    we can find 1,9,21,29,41,...
    or 2,8,22,28,42,...
    or ...
     
  6. Feb 13, 2013 #5

    haruspex

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    Maybe I'm misreading the OP. If a = 0,10,20,30,40,.. is the given sequence, I thought we were looking for other sequences s s.t. a = 0.5( s + s[i+1] ). 1,9,21,29,41,... does not appear to be a solution.
    If we start with an arbitrary s[0] then s[i+1] = 2a - s would appear to generate the rest uniquely. Perhaps it is also required that s is increasing (or maybe non-decreasing)?
     
  7. Feb 14, 2013 #6

    trollcast

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    Yeah thats how the example worked, the new sequence must be non decreasing as well.
     
  8. Feb 16, 2013 #7

    trollcast

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    Actually I looked at this again and for the easiest one, ie. the average sequence 1, 2, 3, 4, ... , theres 2 possible sequences that could have came from:

    0, 2, 2, 4, 4, 6, 6, ...

    or

    1, 1, 3, 3, 5, 5, ...

    I've sat and mucked around with various other sequences and found that something like:

    1, 2, 8, 9 has no sequences for which it could be the average but I can't figure out a way to work this out without going through and working out the possible values for each average sequence?
     
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