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Selective Method Override  
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The chart compares the number of evaluations required to solve a particular ODE with different orders of Runge-Kutta method. Although Mathematica's full automatic algorithm selection produces a near-optimized result, selecting 8th order will in this case be more computationally efficient. However, guessing the best order to use--as required in non-Mathematica, manual-only systems--would typically be highly inefficient.

Setting a selective method override in a Mathematica function means manually narrowing down how Mathematica should carry out the computation to a particular method or class of algorithms, though not its order or sub-algorithm. This is a halfway point between letting Mathematica automatically select the best option using AAS or manually specifying everything.

For example, ExplicitRungeKutta is a selective method override specifiable in the numerical differential equation solver NDSolve. Choosing it enables all the different orders of Runge-Kutta methods to be utilized under AAS control but prevents the use of other methods, such as Adams or BDF.

Related Links
Technology Guide: Automatic algorithm selection, event detection and event localization
Advanced documentation: Advanced Numerical Differential Equation Solving in Mathematica



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