Well-behaved

Mathematicians (and those in related sciences) very frequently speak of whether a mathematical object — a number, a function, a set, a space of one sort or another — is "well-behaved" or not. While the term has no fixed formal definition, it can have fairly precise meaning within a given context.

In pure mathematics, "well-behaved" objects are those that can be proved or analyzed by elegant means to have elegant properties.

In both pure and applied mathematics (optimization, numerical integration, or mathematical physics, for example), "well-behaved" also means not violating any assumptions needed to successfully apply whatever analysis is being discussed.

The opposite case is usually labeled pathological. It is not unusual to have situations in which most cases (in terms of cardinality) are pathological, but the pathological cases will not arise in practice unless constructed deliberately. (Of course, in these matters of taste one person's "well-behaved" vs. "pathological" dichotomy is usually some other person's division into "trivial" vs. "interesting".)

Generally,

*Lebesgue-integrable functions are better-behaved than general functions in calculus.
*Riemann-integrable functions are better-behaved than Lebesgue-integrable functions in calculus.
*Continuous functions are better-behaved than Riemann-integrable functions on compact sets in calculus.
*Continuous functions are better-behaved than discontinuous ones in topology.
*Differentiable functions are better-behaved than general continuous functions. The larger the number of times the function can be differentiated, the more well-behaved it is.
*Smooth functions are better-behaved than general differentiable functions.
*Analytic functions are better-behaved than general smooth functions
*Euclidean space is better-behaved than non-Euclidean geometry.
*Attractive fixed points are better-behaved than repulsive fixed points.
*Fields are better-behaved than skew fields.
*Hausdorff topologies are better-behaved than those in arbitrary general topology.
*Separable field extensions are better-behaved than non-separable ones.
*Borel sets are better-behaved than arbitrary sets of real numbers.
*Spaces with integer dimension are better-behaved than spaces with fractal dimension.
*Spaces with dimension are better-behaved than spaces with infinite dimension in linear algebra.