Two representations of a group and are said to be orthogonal if

for , where the sum is over all elements of the representation.

Two representations of a group and are said to be orthogonal if

for , where the sum is over all elements of the representation.

Weisstein, Eric W. "Orthogonal Group Representations."
From *MathWorld*--A Wolfram Web Resource. https://mathworld.wolfram.com/OrthogonalGroupRepresentations.html