For the Poisson GLMM (9.12) with random intercept, use the normal mgf to show that for j ≠ k, cov(yij, yik) = exp[(xij + xik)????]

[

exp (

????2 u

) (exp (

????2 u

)

− 1

)]

Find corr(yij, yik). Explain why, as in binary GLMMs, corr(yij, yik) ≥ 0.