The lifespan of solutions of semilinear wave equations with the scale invariant damping in one space dimension

The critical constant of time-decaying damping in the scale invari- ant case is recently conjectured. It also has been expected that the lifespan estimate is the same as associated semilinear heat equations if the constant is in "heat-like" domain. In this paper, we point out that this is not true if the total integral of the sum of initial position and speed vanishes. In such a case, we have a new type of the lifespan estimates which is closely related to the non-damped case in shifted space dimensions.