Update README.md

Author: mipals

README.md

@@ -12,9 +12,7 @@ Let $c_j \in \mathbb{C},\ j = 1,\dots,N$ denote a collection of charge strengths
 The Fast Multipole Method is $O(n)$ algorithm that can be used to approximate the Helmholtz potential $u$ (and its gradient and Hessian) caused by the presence of a collection of sources $\mathbf{x}_j$ at target position $\mathbf{x}$. The direct computation of the potential is 
 
 $$
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 u(\mathbf{x}) = \sum_{j=1}^{N} c_jH_0^{(1)}(k\|\mathbf{x} - \mathbf{x}_j\|) - v_j\mathbf{d}_j\cdot\nabla H_0^{(1)}(k\|\mathbf{x} - \mathbf{x}_j\|),
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 $$
 
 where $H_0^{(1)}$ is the Hankel function of the first kind of order 0. When $\mathbf{x} = \mathbf{x}_j$ the $j$ th term is dropped from the sum.