kronecker delta

Summary

Properties

Concept

Kronecker delta

• 1 if the variables are equal
• 0 otherwise

see the similarity with the definition of orthogonality

Identity matrix

• entries match the delta

Generalisation of the dot product

• double sum <-> single sum
• only sum when the components are of the same basis

Application

Infinite sum

sifting: idea of extracting that specific component from the sum