I have a question about generating functions,
I've understood that generating functions are also representing some series of numbers, but is this series could be formed by 2 different functions?
For example, an exercise that I've encountered goes like this:
Find the generating function that represents the number of solutions to the equation:
$$ x_1 + x_2 + \dots + x_k=n$$
Where the variables are even numbers that are not divisible by 3,
So I assume we should find a function that represents the series:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 …
0 1 0 1 0 0 0 1 0 1 0 0 0 1 0 1 0 0 ..
The zero's and one's stands for the coefficient of $x^n$
So to get to that series we have few options the first one i've thought about is to take the generating function: $1 + x + x^2 + x^3 + \dots$
$$ 1 + x + x^2 + x^3 + … = \frac{1}{1-x} $$
Which represents the series of 1 1 1 1 1 …
and then to subtract those (subtract odd numbers and multiples of 6):
$$ f(x) = ((1 + x + x^2 + x^3 + …) – x(1 + x^2 + x^4 + …) – (1 + x^6 + x^{12} +…))^k $$
Which gives us the series we wanted.
But the solution to this exercise shows different answer and the generating function goes like this:
$$ g(x) = ((x^2 + x^4) + (x^8 + x^{10}) + (x^{14} + x^{16}) + … )^k $$
which represents the same series as well but looks differently, does those 2 functions coefficients represent the same number of solutions to the equation?
Thanks alot !
Best Answer
Your approach is fine. We can show the generating functions are equal: $f=g$.
On the other hand we obtain