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Bond percolation between $k$ separated points on a square lattice

We consider a percolation process in which $k$ points separated by a distance proportional to system size $L$ simultaneously connect together ($k>1$), or a single point at the center of a system connects to the boundary ($k=1$), through adjacent connected points of a single cluster. These processes yield new thresholds $\overline p_{ck}$ defined as the average value of $p$ at which the desired connections first occur. These thresholds are not sharp as the distribution of values of $p_{ck}$ for individual samples remains broad in the limit of $L \to \infty$. We study $\overline p_{ck}$ for bond percolation on the square lattice, and find that $\overline p_{ck}$ are above the normal percolation threshold $p_c = 1/2$ and represent specific supercritical states. The $\overline p_{ck}$ can be related to integrals over powers of the function $P_\infty(p)$ equal to the probability a point is connected to the infinite cluster; we find numerically from both direct simulations and from measurements of $P_\infty(p)$ on $L\times L$ systems that, for $L \to \infty$, $\overline p_{c1} = 0.51755(5)$, $\overline p_{c2} = 0.53219(5)$, $\overline p_{c3} = 0.54456(5)$, and $\overline p_{c4} = 0.55527(5).$ The percolation thresholds $\overline p_{ck}$ remain the same, even when the $k$ points are randomly selected within the lattice. We show that the finite-size corrections scale as $L^{-1/ν_k}$ where $ν_k = ν/(k β+1)$, with $β=5/36$ and $ν=4/3$ being the ordinary percolation critical exponents, so that $ν_1= 48/41$, $ν_2 = 24/23$, $ν_3 = 16/17$, $ν_4 = 6/7$, etc. We also study three-point correlations in the system, and show how for $p>p_c$, the correlation ratio goes to 1 (no net correlation) as $L \to \infty$, while at $p_c$ it reaches the known value of 1.022.