prime subfield
The prime subfield of a field is the intersection of all subfields
of , or equivalently the smallest subfield of . It can also be constructed by taking the quotient field of the additive subgroup
of generated by the multiplicative identity
.
If has characteristic where is a prime, then the prime subfield of is isomorphic
to the field of integers mod . When has characteristic zero, the prime subfield of is isomorphic to the field of rational numbers.