# How to write a mathematical model

### Mathematical modeling: problems, methods, applications

Deductive, inductive, or floating: A deductive model is a logical structure based on a theory. In a mathematical programming model, if the objective functions and constraints are represented entirely by linear equations , then the model is regarded as a linear model. Scope of the model[ edit ] Assessing the scope of a model, that is, determining what situations the model is applicable to, can be less straightforward. A model is considered to be nonlinear otherwise. Conversely, in a stochastic modelâ€”usually called a " statistical model "â€”randomness is present, and variable states are not described by unique values, but rather by probability distributions. Physical theories are almost invariably expressed using mathematical models. Well, the formula only makes sense for widths greater than zero, and I also found that for widths above 0. Discrete vs. Often when engineers analyze a system to be controlled or optimized, they use a mathematical model. But wait a minute But sometimes it is the output parameters which are known, and the corresponding inputs must be solved for by an iterative procedure, such as Newton's method if the model is linear or Broyden's method if non-linear. Exogenous variables are sometimes known as parameters or constants. A white-box model also called glass box or clear box is a system where all necessary information is available. For example, if we make a model of how a medicine works in a human system, we know that usually the amount of medicine in the blood is an exponentially decaying function. Example: An ice cream company keeps track of how many ice creams get sold on different days.

It is then not surprising that his model does not extrapolate well into these domains, even though his model is quite sufficient for ordinary life physics. Furthermore, the output variables are dependent on the state of the system represented by the state variables. A model is considered to be nonlinear otherwise.

Fit to empirical data[ edit ] Usually the easiest part of model evaluation is checking whether a model fits experimental measurements or other empirical data.

Model evaluation[ edit ] A crucial part of the modeling process is the evaluation of whether or not a given mathematical model describes a system accurately. But there are still "real world" things to think about, such as "will it be strong enough?

Variables may be of many types; real or integer numbers, boolean values or stringsfor example. Application of mathematics in social sciences outside of economics has been criticized for unfounded models.

This practice is referred to as cross-validation in statistics.

### Stages of mathematical modelling

In engineering , physics models are often made by mathematical methods such as finite element analysis. So the model is useful! Often the a priori information comes in forms of knowing the type of functions relating different variables. Occam's razor is a principle particularly relevant to modeling, its essential idea being that among models with roughly equal predictive power, the simplest one is the most desirable. For example, the de Broglie wavelength of a tennis ball is insignificantly small, so classical physics is a good approximation to use in this case. Bayesian statistics provides a theoretical framework for incorporating such subjectivity into a rigorous analysis: we specify a prior probability distribution which can be subjective , and then update this distribution based on empirical data. Euclidean geometry is much used in classical physics, while special relativity and general relativity are examples of theories that use geometries which are not Euclidean. Practically all systems are somewhere between the black-box and white-box models, so this concept is useful only as an intuitive guide for deciding which approach to take. Subjective information[ edit ] Sometimes it is useful to incorporate subjective information into a mathematical model. For example, in a statistical linear model , it is assumed that a relationship is linear in the parameters, but it may be nonlinear in the predictor variables.

Likewise, he did not measure the movements of molecules and other small particles, but macro particles only. If the model was constructed based on a set of data, one must determine for which systems or situations the known data is a "typical" set of data.

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