Past exam of the mathematics course of the University of Cambridge / 2013 / iii / Paper 23 / 1 / c

For a primitive Dirichlet character the previous formula gives $|\hat{\chi }(a)|=|\chi (a)||\hat{\chi }(1)|$. The Plancherel theorem for the unitary finite transform yieldsThus $|\hat{\chi }(1)|=1$, the normalized Gauss sum of a Dirichlet character magnitude.

For an imprimitive character modulo $p^k$ with $k\ge 2$, its values on residues are periodic modulo $p^{k-1}$: descent preserves the values on units, and divisibility by $p$ is unchanged by that shift. Splitting the sum into these residue classes gives a factor ${\sum }_{j=0}^{p-1}e(-j/p)=0$. Hence $\hat{\chi }(1)=0$. If $k=1$, the only imprimitive character is principal, and its Gauss sum is ${\sum }_{n=1}^{p-1}e(-n/p)=-1$, giving $|\hat{\chi }(1)|=p^{-1/2}$. All cases satisfy the required bound.