Darboux sums and Riemann integrability
For bounded real f on [a,b], lower and upper Darboux sums use each subinterval's infimum and supremum.
Key knowledge
For bounded real f on [a,b], lower and upper Darboux sums use each subinterval's infimum and supremum. The Riemann integral exists when the supremum of lower sums equals the infimum of upper sums; equivalently, for every ε>0 some partition has U(f,P)−L(f,P)<ε.
Values and equations
L(f,P)=Σᵢ(inf_[xᵢ₋₁,xᵢ] f)(xᵢ−xᵢ₋₁); U(f,P)=Σᵢ(sup_[xᵢ₋₁,xᵢ] f)(xᵢ−xᵢ₋₁)
- f
- Bounded real-valued function on the finite interval
- P
- Finite partition with strictly increasing endpoints xᵢ
- L,U
- Lower and upper Darboux sums
Conditions and scope
- Use finite a<b and finite partitions a=x₀<x₁<⋯<xₙ=b; infima and suprema need not be attained.
- This is bounded proper Riemann integration, not an improper limit or a condition on just one fixed partition.
Sources · 1
References
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Evidence and review
Section 11.2 Definition 11.11, printed pages 208–210; Theorem 11.23 and both directions of its common-refinement proof, printed page 215.
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