What does this symbol mean in this equation?

Question

I am reading up on n-choose-k problems (Binomial Coefficients).

Wikipedia gives a multiplicative solution that is more efficient:

I've taken a few calculus courses, and it reminds me of how you would use the summation symbol ∑, but it also kind of looks like pi (π). What is this symbol and how can I use it to understand this formula?

Answer 1

$\prod$ means product just as $\sum$ means sum. So, $\prod_{i=1}^5 i = 1 \cdot 2 \cdot 3 \cdot 4 \cdot 5$

Most often, it is defined as: $$ \prod_{i=0}^0 a_i = a_0 $$ $$ \prod_{i=0}^n a_i = (\prod_{i=0}^{n-1} a_i)\cdot a_n$$

Answer 2

$\prod_{k=1}^{n} k = 1 \cdot 2 \cdot \ldots n$, i.e. a product sighn

Answer 3

Pi as in product:

$\prod_{k=1}^n a_k=a_1\cdot a_2\cdots a_{n-1}\cdot a_n$.