In a poisson distribution, the variance is m. The sum of terms in odd places in the distribution is
e−m
e−mcosh(m)
e−msinh(m)
e−mcoth(m)
λ=m
Sum of Odd terms =P(X=0)+P(X=2)+P(X=4)+...........
=e−m[1+m22!+m44!+..........]
=e−m2[[1+m1!+m22!+m33!+...........]+[1−m1!+m22!−m33!+...........]]
=e−m[em+e−m2]=e−mcosh(m)