The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all() error in a if statement

Question

I\'m trying to work these comand lines:

for lm_inf in df.columns:
    if df[\'a1\'].astype(\'int64\') > 0 & df[\'b1\'].astype(\'int64\') < 0: