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Let $v_1:\mathbb{P}^1 \longrightarrow \mathbb{P}^2$ and $v_2:\mathbb{P}^1 \longrightarrow \mathbb{P}^2$ be two holomorphic maps of degree $d_1$ and $d_2$ respectively. Suppose they agree at some point (say $[1,0]$), i.e. $$ v_1([1,0]) = v_2([1,0]).$$ Then we can think of this whole collection as a stable map $v$ from a nodal riemann surface (the two spheres identified at $[1,0]$). Suppose $v_1$ and $v_2$ are explicitly given as $$v_1([X,Y]):= [p_0(X,Y), p_1(X,Y), p_2(X,Y)],$$ $$ v_2([X,Y]):= [q_0(X,Y), q_1(X,Y), q_2(X,Y)],$$ where $p_i$ and $q_i$ are homogeneous polynomials of degrees $d_1$ and $d_2$ respectively.

$\textbf{Question:}$ Can one explicitly construct a sequence of holomorphic maps $u_t:\mathbb{CP}^1\rightarrow \mathbb{CP}^2$ of degree $d_1+d_2$ that converges to the stable map $v$? I am looking for an explicit formula for the family $u_t$ in terms of the polynomials $p_i$ and $q_i$ and $t$.

The convergence is in the sense of Gromov Gonvergence for $J$-holomorphic maps.

$\textbf{Remark:}$ I am aware that abstractly the $u_t$ exists; it follows from general gluing theorem for $J$-holomorphic maps. I am just wondering in the special case of degree $d$ rational maps in $\mathbb{CP}^2$ if one can explicitly write down that family of maps $u_t$ in terms of polynomials.

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  • $\begingroup$ One of the ways is to find a map $P^1\times P^1 \to P^2$ such that on $P^1\times \{0\}$ it agrees with the first map and on $\{0\}\times P^1$ with the second. Then just restrict this map to the family of curves $x_1y_1 = tx_0y_0$ in $P^1\times P^1$. $\endgroup$
    – Sasha
    Commented Apr 28, 2015 at 9:57
  • $\begingroup$ @Sasha: This is a brilliant idea! I am just wondering what would be the difficulty in carrying out the idea for a more general symplectic manifold M as opposed to P^2; is it difficult to find a map from P^1 X P^1 that agrees on P^1X{0} and {0}XP^1 with two given maps? Your idea should at least work if M is algebraic (sitting inside some projective space). $\endgroup$
    – Ritwik
    Commented Apr 28, 2015 at 10:26

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