If(aλ)λ∈Lis a summable family of real numbers and if(xμ)μ∈Mis a summable family of points ofRn, then the family(aλxμ)(λ,μ)∈L×Mis summable and we have
(λ,μ)∈L×M∑aλxμ=(λ∈L∑aλ)μ∈M∑xμ.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.