This is an immediate consequence of Vieta's formulas. In fact, the th roots of unity being the roots of the polynomial , their sum is the coefficient of degree , which is either 1 or 0 according whether or .
This is an immediate consequence of Vieta's formulas In fact the th roots of unity being the roots of the polynomial  their sum is the coefficient of degree  which is either 1 or 0 according whether or 