Let
Y be a smooth curve embedded in a complex projective manifold
X of dimension
n&\ge;2 with ample normal bundle
NY|X. For every
p&\ge;0 let α
p denote the natural restriction maps Pic(
X)→Pic(
Y(
p)), where
Y(
p) is the
p-th infinitesimal neighbourhood of
Y in
X. First one proves that for every
p&\ge;1 there is an isomorphism of abelian groups Coker(α
p)$\cong$Coker(α
0)$\oplus$
Kp(
Y,X), where
Kp(
Y,X) is a quotient of the
C-vector space
Lp(
Y,X):=$\bigoplus$
i=1p H1(
Y,
Si(
NY|X)
*) by a free subgroup of
Lp(
Y,X) of rank strictly less than the Picard number of
X. Then one shows that
L1(
Y,X)=0 if and only if
Y$\cong$
P1 and
NY|X$\cong\mathscr{O}_{\bm{P}^1}(1)^{\oplus n-1}$ (i.e.
Y is a quasi-line in the terminology of [
4]). The special curves in question are by definition those for which dim
CL1(
Y,X)=1. This equality is closely related with a beautiful classical result of B. Segre [
25]. It turns out that
Y is special if and only if either
Y$\cong$
P1 and
NY|X$\cong\mathscr{O}_{\bm{P}^1}(2)\oplus\mathscr{O}_{\bm{P}^1}(1)^{\oplus n-2}$, or
Y is elliptic and deg(
NY|X)=1. After proving some general results on manifolds of dimension
n≥2 carrying special rational curves (e.g. they form a subclass of the class of rationally connected manifolds which is stable under small projective deformations), a complete birational classification of pairs (
X,Y) with
X surface and
Y special is given. Finally, one gives several examples of special rational curves in dimension
n≥3.
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