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Characteristic

The term "characteristic" has many different uses in mathematics. In general, it refers to some property that inherently describes a given mathematical object, for example characteristic class, characteristic equation, characteristic factor, etc. However, the unqualified term "characteristic" also has a number of specific meanings depending on context.

For a real number x, |_x_|=int(x) is called the characteristic, where |_x_| is the floor function.

A path in a two-dimensional plane used to transform partial differential equations into systems of ordinary differential equations is also called a characteristic. This form of characteristic was invented by Riemann. For an example of the use of characteristics, consider the equation

 u_t-6uu_x=0.

Now let u(s)=u(x(s),t(s)). Since

 (du)/(ds)=(dx)/(ds)u_x+(dt)/(ds)u_t,

it follows that dt/ds=1, dx/ds=-6u, and du/ds=0. Integrating gives t(s)=s, x(s)=-6su_0(x), and u(s)=u_0(x), where the constants of integration are 0 and u_0(x)=u(x,0).

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