Information flow and the structure of design problems

The general design problem can be represented in mathematical symbolism as a series of equations  

There are seven variables, Nv = 7, and five equations (relationships) Nr = 5, so the number of degrees of freedom is  

The task is to select two variables from the total of seven in such a way as to give the simplest, most efficient, method of solution to the seven equations. There are twenty-one ways of selecting two items from seven. 

The variables selected as design variables (fixed by the designer) cannot therefore be assigned as output variables from an f node. They are inputs to the system and their edges must be oriented into the system of equations. 

Knowing v6 and V5, fs can be solved to give v2 which completes the solution. 

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