# Please help to find term's coefficient in the following example

I trying find the number of all solutions in the following:

$x_1 + x_2 + x_3 + x_4 + x_5 = 24$

where:

2 of variables are natural odd numbers

3 of variables are natural even numbers

none of variables are equal to $0$ or $1$ (all the variables are $>= 2$)

So I've made generating functions according to the limitations as follow:

$(x^3 + x^5 + x^7 + ... + x^{21} + x^{23})^2$ $(x^2 + x^4 + x^6 + ... + x^{22} + x^{24})^3$

Now I need to find the coefficient of $x^{24}$

I've tried to solve it but get stuck, here is what I've done:

$x^6(1 + x^2 + x^4 + ... + x^{18} + x^{20})^2$ $x^6(1 + x^2 + x^4 + ... + x^{20} + x^{22})^3$

$x^{12}(1 + x^2 + x^4 + ... + x^{18} + x^{20})^2(1 + x^2 + x^4 + ... + x^{20} + x^{22})^3$

Now we actually looking for coefficient of $x^{24-12}=x^{12}$ in:

$(1 + x^2 + x^4 + ... + x^{18} + x^{20})^2(1 + x^2 + x^4 + ... + x^{20} + x^{22})^3$

$(\frac{1 - x^{21}}{1 - x})^2(\frac{1 - x^{23}}{1 - x})^3$

Here I'm stuck. How to proceed in order to find coefficient of $x^{12}$?

Regards.

PS: Solution was given but just out of curiosity, how to solve it using generating-functions?

Let the numbers be $2a + 1, 2b + 1, 2c, 2d, 2e$, where $a, b, c, d, e$ are natural. Note that this makes the odd numbers greater than or equal to $3$, and the even numbers greater than or equal to $2$.

Then you have $2a + 1 + 2b + 1 + 2c + 2d + 2e = 24 \rightarrow a + b + c + d + e = 11$. You can solve this with a standard stars-and-bars approach.

• That's very nice! But just out of curiosity, how to solve it using generating-functions? – Mike Jun 15 '15 at 17:13
• Doesn't this approach suffer from positioning the two odd summands first and the even summands last, a restriction not imposed in the Question? – hardmath Jun 15 '15 at 19:10
• @hardmath You bring up a good point. It looks like the numbers can be in any order, so I guess we can just multiply by $\binom{5}{2}$ in the end? – Ashkay Jun 15 '15 at 19:13
• Maybe the rearrangements are built in to your calculation. If you take $\binom{10}{4}$ in stars-and-bars, you get $210$ like I got. I'm a bit confused by this. – hardmath Jun 15 '15 at 19:28
• @hardmath isn't that should be $\binom{11+5-1}{11}$ so instead you get $\binom{15}{11} = 1365$ – Mike Jun 15 '15 at 19:53

Let's take it from the point where we ask for the coefficient of $x^{12}$ in:

$$(1 + x^2 + x^4 + ... + x^{18} + x^{20})^2(1 + x^2 + x^4 + ... + x^{20} + x^{22})^3$$

[Note, however, that a factor of $\binom{5}{2}=10$ is already left out of the calculation at this point, which would account for varying positions of the two odd summands $x_i$. See the comments on Ashtay's Answer for more details.]

Replace $x^2$ with $y$ and ask for the coefficient of $y^6$:

$$(1 + y + y^2 + \ldots + y^9 + y^{10})^2 (1 + y + y^2 + \ldots + y^{10} + y^{11})^3$$

Of course we can remove all polynomial in $y$ terms with exponents greater than $6$ since these cannot contribute to the coefficient of $y^6$, then notice the polynomial factors are now equal:

$$(1 + y + y^2 + \ldots + y^6)^5$$

We can continue to explicitly work out the coefficient of $y^6$ by repeated squarings (and removing terms beyond $y^6$):

$$(1 + y + \ldots + y^6)^2 \; \to \; (1 + 2y + 3y^2 + 4y^3 + 5y^4 + 6y^5 + 7y^6)$$

$$(1 + 2y + 3y^2 + \ldots + 7y^6)^2 \; \to \; (1 + 4y + 10y^2 + 20y^3 + 35y^4 + 56y^5 + 84y^6)$$

Multiplication by $(1+y+y^2+\ldots + y^6)$ of the last result gives the desired term $210y^6$. It follows that $210$ was the original coefficient of $x^{24}$ you wanted (if my arithmetic is right!).

• Thanks a lot, it looks very good! Is there any other (less tedious) way to get the desired coefficient of $y^6$ out of $(1+y+y^2+y^3+y^4+y^5+y^6)^5$? – Mike Jun 15 '15 at 19:30
• The method of @Ashkay, i.e. stars-and-bars, works on that as well and reduces it to computing the sums of $5$ nonnegative integers that add up to $6$, $\binom{6+4}{4} = 210$. – hardmath Jun 15 '15 at 19:46