1. A miniature digital computer circuit comprising: first and second sources of digital data signals respectively representing a first function having the format X(t), where X(t) is a function of t which is a continually varying first quantity, and a second function having the format Y(t), where Y(t) is a second function of t which is a continually varying second quantity, said digital data signals having the format of two trains of successive groups of pulses, said groups of pulses in each of said trains having a prescribed number of bits; first and second pulse readout memory means connected to said first and second data signal sources and arranged to produce first and second partial derivative digital readouts having the respective formats .differential.F(X,Y)/.differential.X and .differential.F(X,Y)/.differential.Y, where .differential.F(X,Y)/.differential.X and .differential.F(X,Y)/.differential.Y represent the partial derivatives of the function F(X,Y) taken only with respect to said first and second functions, and .differential.X, .differential.Y represent the partial differentials of said first and second varying quantities; first digital multiplier means connected to said first memory means and said first data signal input source for receiving therefrom and multiplying said first partial derivative of digital readout data pulses by .DELTA.X, where .DELTA.X represents the instantaneous change in quantity X from instant to instant, to produce a first digital multiplication product having the format .differential.F(X,Y)/.differential.X .DELTA.X; second digital multiplier means connected to said second memory means and said second data signal input source for multiplying said second partial derivative digital readout data pulses by .DELTA.Y, where .DELTA.Y represents the instantaneous change in said second quantity from instant to instant, to produce a second digital multiplication product having the format .differential.F(X,Y)/.differential.Y .DELTA.Y; and digital adder means connected to said first and second multiplier means to receive said digital multiplication products therefrom and to add said products together to produce a digital sum having the format .DELTA.F(X,Y), where .DELTA.F(X,Y)=.differential.F(X,Y)/.differential.X .DELTA.X+.differential.F(X,Y)/.differential.Y .DELTA.Y, and where .DELTA.F(X,Y) represents the two first order terms of the partial derivative expansion of the function F(X,Y).