)Since the expectation of an unbiased estimator "δ"("X") is equal to the estimand, i.e.the only function of the data constituting an unbiased estimator is

To see this, note that when decomposing e−"λ" from the above expression for expectation, the sum that is left is a Taylor series expansion of e−"λ" as well, yielding e−"λ"e−"λ" = e−2"λ" (see Characterizations of the exponential function).