Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Period of superposed cyclic integer rows

  1. Mar 13, 2010 #1
    Take two rows of respective length m and n:
    a1, a2, a3,..., am and b1, b2, b3, ..., bn.

    Then produce as follows the generated array Gai to contain these elements:
    a1, a1+a2, a1+a2+a3, ..., a1+..+am,
    a1+..+am+a1, a1+..+am+a1+a2, .....

    Alike produce the generated array Gbj to contain these elements:
    b1, b1+b2, b1+b2+b3, ..., b1+..+bn,
    b1+..+bn+b1, b1+..+bn+b1+b2, ....

    The numbers ai and bj are cumulated cyclically to produce their respective arrays Gai and Gbj.

    Two questions are open to be analyzed (by me) - hope someone has a hint:
    1. How to express analitically all numbers contained in Gai U Gbj as a function.
    2. Since the rows ai and bj has periods m and n respectively, what is the resulting period of the superposition , ie. the period of Gai U Gbj?

    I appreciate your comment or hint.
  2. jcsd
  3. Mar 14, 2010 #2
    looks to me like you have an infinite set.
  4. Mar 14, 2010 #3
    Yes sure, the numbers are going infinite, but the indicator function on the set ai has periodic behaviour. That is what I am aiming at.

    To illustrate take the following two simple sets to begin with:
    Let ai = { 7, 4, 7, 4, 7, 12, 3, 12 }
    and bj = { 12, 6, 11, 6, 12, 18, 5, 18 }

    whereby the numbers in the set are cumulated round robin, as described in my first post. So the indicator funtion has a 1 on postion 7, 11, 18, 22, 29, 41, 44, 56, 56+7, 56+7+4, etc etc.

    The same for the bjs : the indicator has a non-zero (=1) on 12, 18, 29, 35, 47, 65, 70, 88, 88+12, 88+12+6, etc. etc.

    So obviously the indicator on both sets ai and bj separately, are periodic (periodicity = 8). So the superposition of both periodical sets have a periodicity ( like a discrete Fourier).

    The first question would be what that period looks like and how it is calculated. It can be done heuristically with excel sheets, but that is unsatisfactory.
    Also, what does the superposed indicatorset looks like.

    Having these two superposed, the a third set of ck would be superposed with the first superposition - and so on.

    That is where my (more generalized) initial question arose from.
  5. Mar 14, 2010 #4
    Now that I understand you more, I would think the period will repeat after the indicator element=1 at 8*7*11 since 56 = 8*7 and 88 = 8*11 and have a period as high as 8*(7+11) elements not excluding any duplicate occurrences of the same integer in each set. See if that compares favorably with your spread sheet.
  6. Mar 15, 2010 #5

    Thank you - that covers my results - The Question I am most eager to get answered is: how to 'walk through the ONEs' of the combined indicator function. That is, a function walking me through the ONEs, or better even the other way around, walk me through the ZEROs. I think this should be could be done by a discrete Fourier, but am puzzling how to do this best.
  7. Mar 15, 2010 #6
    To answer part of your question, the 'period' (of the index, in the restricted sense you have used, since the sequence itself is not periodic) of the combination will be http://en.wikipedia.org/wiki/Least_common_multiple" [Broken](m,n).

    I suppose you have noticed that you only need to describe any particular sequence, say Ga, only up to the length m of the array: if you divide any possible index x by m, to produce a quotient q and a residue r (x = qm+r), then Gax = q Gam + Gar.
    Last edited by a moderator: May 4, 2017
  8. Mar 16, 2010 #7
    absolutely true - this is the periodicity or the modulus. Since multiple 'rows' with their own modulus are superposed, the question still is how to formulate the superposition at number n.
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook