How many regions of the plane are bounded by the graph of x6−x5+3x4y2+10x3y2+3x2y4−5xy4+y6=0?
A number or a short expression. Spacing and $ signs are ignored.
Solution
The left-hand side decomposes as (x6+3x4y2+3x2y4+y6)−(x5−10x3y2+5xy4)=(x2+y2)3−(x5−10x3y2+5xy4). Now, note that (x+iy)5=x5+5ix4y−10x3y2−10ix2y3+5xy4+iy5 so that our function is just (x2+y2)3−ℜ((x+iy)5). Switching to polar coordinates, this is r6−ℜ(r5(cosθ+isinθ)5)=r6−r5cos5θ by de Moivre's rule. The graph of our function is then the graph of r6−r5cos5θ=0, or, more suitably, of r=cos5θ. This is a five-petal rose, so the answer is 5.
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