define

μi+1=∫−∞tμi(λ−t)dλ\mu_{i+1} = \int_{-\infty}^t \mu_i(\lambda - t) d\lambda

where μ0(t)=δ(t)\mu_0(t) = \delta(t), i.e. δ-function or unit impulse,

and, μ1(t)=1(t)\mu_1(t) = 1(t), i.e. Step function

References

  1. Sarachik - Principles of linear systems, 1997, p 43
  2. https://en.wikipedia.org/wiki/Singularity_function