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Stirling numbers of a second kind, from the hint in the book, look correct?

  1. Nov 23, 2006 #1
    Hello everyone i was wondering if someone could check to see if this looks correct or not. The question is:

    If X is a set with n elements and Y is a set with m elements, express the number of onto functions from X and Y using Stirling numbers of the second kind. Justify your answer.

    note: the x in the latex generated graphics means multiply not the variable 'x'.

    for more info on stirling numbers of a second kind look here:

    Well the back of the book gave the following hint:
    The number of onto functions from X = {x_1,x_2,x_3,x_4} to Y = {y_1,y_2,y_3} is [tex]S_{4,3}x3![/tex] because the construction of an onto function can be thought of as a 2-step processs where
    step 1: is to choose a partion of X into 3 subsets and
    step 2: is to choose, for each subset of the partition, an element of Y for the elements of the subset to be sent to.

    Well in the question X has n elements in the hint 4 elements.
    Y has m elements in the question and in the hint it has 3 elements.
    So it looks like the answer would be:
    [tex]S_{n,m} x m![/tex]
    does that look right to you?

    Last edited: Nov 23, 2006
  2. jcsd
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