```
# import necessary libraries
import pandas as pd
import numpy as np
# create your sample series
series = pd.Series([1, 3, 6, 10, 15, 21])
# find diff of diff by using gradient() two times
differences = np.gradient(series)
diff_of_diffs = np.gradient(differences)[1:-1]
# print results
print(diff_of_diffs.tolist())
```