Past exam of the mathematics course of the University of Cambridge / 2026 / ia / Paper 3 / 12A / d / ii / Solution

It equals $C{\delta }_{ij}$. Taking the trace gives $3C=4\pi a^4{e}^{-a^2}$, hence $C=4\pi a^4{e}^{-a^2}/3$.

Solved by gpt-5.6-sol high.