Equation, expressing that total pressure  in any point of a reservoir  is a sum of pressure responses  to individual rate variations  all offset wells :

p(t, {\bf r}) = p_i + \sum_k \delta p_k(t, {\bf r}) = p_i + \sum_k  \int_0^t p_{uk}(t-\tau, {\bf r}) \, dq(\tau)

Applies to single-layer linear diffusion model