Principal root

The square roots of 4 are 2 and −2; the principal square root of 4 is 2.

The 3rd roots of −8 are 1 + i√3, −2 and 1 − i√3; the principal 3rd root of −8 is 1 + i√3 and the real root is −2.

The 4th roots of 16 are 2, −2, 2i and −2i; the principal 4th root of 16 is 2.