Sunday, 10 December 2017

Generating Functions for an:n=i1+i2+i3+i4

I'm having an intro combinatorics class and I do understand the theorems but I don't really know how to apply them. It's pretty easy to solve straight forward questions, but I just don't understand how to solve harder questions, can someone explain this question to me? What I got is completely different from the back of the book ... should I drop this class?



Let an be the number of ways to write the number n as i1+i2+i3+i4, where i1,i2,i3,i4 are integers such that:



$$00\\i_3=2 \text{ or } 4\\i_4>1$$



How do I write the generating function for this sequence, representing this as a rational function?




The only thing I know to first solve this question is to solve it by using the multiplication rule of power series (perhaps?). I know the generating function is a product of 4 power series, corresponding to i1,i2,i3,i4.



Help appreciated. Really struggling.

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