不錯的函數(shù)
它的哲學是

易于上手(很快就到了一個高度),難于精通(就是到不了1)

做條目推薦可以作為加權函數(shù)

In [89]: def aaa(b):return 2/(1+2.71828**-(float(b)/10))-1
   ....:

In [90]: aaa(1)
Out[90]: 0.049958341409185536

In [91]: aaa(0)
Out[91]: 0.0

In [92]: aaa(5)
Out[92]: 0.24491850432781681

In [93]: aaa(10)
Out[93]: 0.46211689275686596

In [94]: aaa(20)
Out[94]: 0.76159387345873797

In [95]: aaa(30)
Out[95]: 0.90514807131548203

In [96]: aaa(40)
Out[96]: 0.9640274850287549

In [97]: aaa(50)
Out[97]: 0.98661425343302223

In [98]: aaa(60)
Out[98]: 0.99505473377744025