Many-to-one correspondence
A many-to-one correspondence is a function or mapping that takes the same value for at least two different elements of its domain. For example, the squaring function 
is many-to-one because 
for all real numbers. 
.
A many-to-one correspondence is a function or mapping that takes the same value for at least two different elements of its domain. For example, the squaring function is many-to-one because for all real numbers. .