one-to-one in a sentence

1. One-to-one onto function is also called bijection .
2. Two sets are said to be equivalent if and only if they can be placed in one-to-one correspondence .
3. It is legitimate to envisage a more or less one-to-one correspondence between cloud nucleus concentration and the concentration of cloud droplets .
4. It's difficult to find one-to-one in a sentence.