The '''Lebesgue number''' of an [[open cover]] ''C'' of a metric space ''(X, d)'' is a real number <math>\delta > 0</math> such that if a subset ''A'' of ''X'' has diameter less than <math>\delta</math>, then ''A'' is contained in at least one element of ''C''. If ''X'' is [[compact space|compact]], then it has at least one Lebesgue number. | The '''Lebesgue number''' of an [[open cover]] ''C'' of a metric space ''(X, d)'' is a real number <math>\delta > 0</math> such that if a subset ''A'' of ''X'' has diameter less than <math>\delta</math>, then ''A'' is contained in at least one element of ''C''. If ''X'' is [[compact space|compact]], then it has at least one Lebesgue number. |