Verified Note

A Weighting Argument for Pointwise Convergence to Convergence in Measure

A measurable weight suppresses the pointwise envelope of a convergent sequence so that pointwise or almost-everywhere convergence becomes convergence in measure.

Summary

Result and mechanism

The note presents a weighting mechanism in two forms: an Egorov-style countable decomposition and a shorter dominated-convergence proof.

Main result

For a sequence of real-valued measurable functions converging to zero almost everywhere on a sigma-finite measure space, there is a conull measurable set A and a strictly positive measurable weight h such that f_n h converges uniformly to zero on A, and therefore converges to zero in measure. The Lean file also records the real-line specialization for Lebesgue measure.

Contribution type
New construction; Lean formalization
Contribution
Explicit measurable-weight construction upgrading almost-everywhere convergence on a sigma-finite measure space to conull uniformization and convergence in measure after multiplying by a strictly positive measurable weight.
Core mechanism
Egorov pieces, pointwise envelope, measurable cover-index weight, convergence in measure
Lean theorem
PointwiseWeighting.exists_conull_pos_measurable_weight_uniformOn_tendstoInMeasure_of_ae_tendsto
Real-line theorem
PointwiseWeighting.real_exists_conull_pos_measurable_weight_uniformOn_tendstoInMeasure
Dominated route
PointwiseWeighting.exists_pos_measurable_weight_tendstoInMeasure_of_ae_tendsto
Lean and mathlib
Lean 4.29.1, mathlib v4.29.1
Build command
lake build