Chapter 7 Local properties of plane algebraic curves - RISC

mult O (e ∩ y) = (4),(7) mult O (x 4 ∩ y) = (6),(5) 4,
mult O (e ∩ h) = (5) mult O (e) · mult O (h) = 6.
Thus, mult O (E, F) = mult O (e ∩ f) = 14.
Theorem 7.2.4. The line l is tangent to the curve f at the point P if and only if
mult P (f ∩ l) > mult P (f).
Proof: By the property (5), l is tangent to f at P if and only if mult P (f ∩ l) >
mult P (f) · mult P (l) = mult P (f).
Theorem 7.2.5. If the line l is not a component of the curve f, then
∑
P ∈A 2 mult P (f ∩ l) ≤ deg(f).
Proof: Only points in f ∩ l can contribute to the sum. Let l be parametrized as
{x = a + tb, y = c + td}. Let
g(t) = f(a + tb, c + td) = α ·
r∏
(t − λ i ) e i
,
for α, e i ∈ K. g ≠ 0, since l is not a component of f.
P ∈ f ∩ l if and only if P = P i = (a + λ i b, c + λ i d) for some 1 ≤ i ≤ r.
If all the partial derivatives of f of order n vanish at P i , then by the chain rule also
∂ n g
∂t n (λ i) =
So mult Pi (f) ≤ e i .
Summarizing we get
i=0
i=1
n∑
( ) n ∂ n f
i ∂x i ∂y n−1(a + λ ib, c + λ i d) · b i · d n−i = 0.
∑
P ∈A 2 mult P (f∩l) =
r∑
mult Pi (f∩l) = (5)
i=1
r∑
i=1
mult Pi (f)·mult Pi (l)
} {{ }
=1
≤
r∑
e i ≤ deg(f).
i=1
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