词频处理 tf/idf.
协同过滤
1.Co-occurence matrix. Normalize co-occurrence Similarity matrix.
1.1 Jaccard similarity: normalizes by popularity
Who purchased i and j divided by who purchased i or j
$\frac{purchase i and i}{purchase i or j}$
原文：https://www.cnblogs.com/Shinered/p/9029872.html