PMKielstra

import numpy as np
from matplotlib import pyplot as plt
from tqdm import tqdm

def K(r1, r2, k):
    """e^{k pi i |r1 - r2|} / |r1 - r2|"""
    return np.exp(1j * k * np.pi * np.linalg.norm(r1 - r2)) / np.linalg.norm(r1 - r2)

def interaction_tensor_matrix(N, C, l):
    """C is the width of the lowest element of the quadtree; N is the width of the overall tensor."""