I'm trying to find the coefficient of:
$$j^2k^3lm^3$$
in:
$$(j + k + l + m)^9$$
According to the Book of Proof (which is our material), it seems for:
$$x^ay^b, \{a,b\} \in \mathbb{N}$$
in:
$$(x+y)^c, c \in \mathbb{N}, c \geq \{a,b\}$$
we can use the binomial theorem:
$$(x + y)^n = \binom{n}{0}x^n + \binom{n}{1}x^{n-1}y + \binom{n}{2}x^{n-2}y^2 + ... + \binom{n}{n-1}xy^{n-1} + \binom{n}{n}y^n$$
However, I'm lost on the other one. Please help?