While Zee obtains the momentum space normalization [ a ^ ^ p , a ^ ^ q † † ] = δ δ ( p − − q ) via the symmetric convention for Fourier transforms, Tong and Peskin & Schroeder use the common asymmetric convention to obtain [ a ^ ^ p , a ^ ^ q † † ] = ( 2 π π ) 3 δ δ ( p − − q ) . Each derives [ ϕ ϕ ^ ^ ( x ) , π π ^ ^ ( x ′ ) ] = i δ δ ( x − − x ′ ) .