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

Repeating sequence

  1. Feb 7, 2012 #1
    1. The problem statement, all variables and given/known data

    The sequence is 1 1 1 1 5 5 5 5 1 1 1 1

    I need to find the formula for the sequence.

    2. Relevant equations

    3. The attempt at a solution

    I had a previous problem that was similar. It was a sequence of 1 5 1 5 1 5. I managed to get it with the formula of 3+2(-1)n.

    I did a little research through my notes/book and I found an explanation. It said I could set n equal to a fraction to get it to alternate numbers for different lengths. It was implying that (-1)n/2 would give me 2 repeating of the number from the equation, (-1)n/4 would give me 4 repeating, etc.

    I understand the concept, at least, I think I do, but when I punch it into my calculator, (-1)n/any# gives me an error. I know (-1)n/# is the same as the # root of the number, and the root of -1 is an imaginary number.

    Maybe I misunderstood something in the notes. Anyone point me in the right direction? Thanks.
  2. jcsd
  3. Feb 8, 2012 #2
    The hint with (-1)^(n/2) only works if you use the Gauss floor function, because, as you said correctly (-1)^(1/2)=i, hence not what you are looking for.

    Try a+b*(-1)^n+c*(-1)^(2n)+d*(-1)^(3n)+e*(-1)^(4n)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook