y = -\frac12

\mathbb{S}

(0,1)

(0,0)

\longrightarrow\
\longrightarrow\
\longrightarrow

\longrightarrow\
\longrightarrow\
\longrightarrow

\longrightarrow\
\longrightarrow\
\longrightarrow

\longrightarrow\
\longrightarrow\
\longrightarrow

\longleftarrow\
\longleftarrow\
\longleftarrow

f: y = -1 / 2
e(x, y) = x^(2) + (y - 1 / 2)^(2)  <  1 / 4
a(x, y) = x^(2) + (y - 1 / 2)^(2) > 1 / 4
c: x^(2) + (y - 1 / 2)^(2) = 1 / 4
A = Point(c)
B = Point(c)