Denote the input of a system by h ( t ) (e.g. a force), and the response of the system by x ( t ) (e.g. a position). Generally, the value of x ( t ) will depend not only on the present value of h ( t ) , but also on past values. Approximately x ( t ) is a weighted sum of the previous values of h ( t ′ ) , with the weights given by the linear response function χ χ ( t − − t ′ ) : x ( t ) = ∫ ∫ − − ∞ ∞ t d t ′ χ χ ( t − − t ′ ) h ( t ′ ) + ⋯ ⋯ .