Perturbative series expansion for the gap of the evolution operator associated with the contact process

The gap between the eigenvalues of the dominant and subdominant eigenvectors of the evolution operator associated with the contact process in one dimension is studied by perturbative series expansions in powers of the creation rate. The series expansion for the gap was computed with 49 coefficients. An analysis by Pade approximants allowed the determination of the critical creation rate and the time correlation critical exponent.