Limitations when entering equations

Functions must be Y=f(X)

When you enter an equation into Prism, the independent variable must be 'X' and the dependent variable must be 'Y'. So if you measure a voltage as a function of time, you cannot enter an equation that defines V as a function of t. It must define Y as a function of X.

Models Prism cannot fit

Prism gives you a lot of flexibility when entering equations, but you must beware of these limitations:

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No implicit equations. Y must be defined as a function of X and one or more parameters. The variable Y can only appear once, on the left side of the last line of the equation. If Y also appears on the right side of the equation, you have an implicit equation, which Prism cannot handle. In many cases, you will be able to algebraically rearrange the equation.

•	No differential equations.You must define Y as a function of X and one or more variables. It is not sufficient to define the derivatives.

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No models with more than one X variable. Prism does not calculate multiple regression, so cannot fit models with two or more independent (X) variables. But note that you can define a parameter to be a column constant, in which case its value comes from the column titles. In some cases, you can think of these column constants as being a second independent variable.

Model complexity

Prism compiles your equation into an internal format it uses to calculate the math efficiently. If the compiled version of your equation won't fit in the space Prism sets aside for this purpose, it reports that the equation is "too complex" .

If you see this message, don't give up. You can usually rewrite an equation to make it less complex. Do this by defining an intermediate variable that defines combinations of variables. For example if your equation uses the term "K1+K2" four times, you reduce complexity (but keep exactly the same mathematical meaning) by defining an intermediate variable at the top of your equation (say, K1P2=K1+K2) and then using that intermediate later in the equation. That way Prism has fewer steps to store.