It looks like the 'b' value is the number of sections the curve is divided into. Also, I noticed that when the value of 'a' is even, the resulting graph is open and when 'a' is odd, the graph is closed.
Let's try a few more and see if this is the case:
When a = 1 and b = 3, the following graph results:
*Notice: the curve is closed and is split into 3 sections.
When a = 1 and b= 4, the following graph results:
*Notice: the curve is closed and is split into 4 sections.
When a = 3 and b = 1, the following graph results:
*Notice: the curve is closed and is split into 3 sections.
When a = 4 and b = 1, the following graph results:
*Notice: the curve is open ...
OH ... WELL! My observation was WRONG!!!