Riemann Integrabe Step Functions

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SUMMARY

The discussion focuses on the Riemann integrability of the composition of functions, specifically when one of the functions, either g or f, is a step function. It establishes that if g: [a,b] → [c,d] is Riemann integrable and f: [c,d] → ℝ is also Riemann integrable, then the composition f ∘ g is Riemann integrable on [a,b] if at least one of the functions is a step function. The participants highlight the straightforward nature of proving this when g is a step function, while noting the complexity involved when f is a step function. The discussion also references the formal composition of characteristic functions and their extension by linearity.

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Arkuski
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Suppose that g:[a,b]\rightarrow[c,d] is Riemann integrable on [a,b] and f:[c,d]\rightarrow ℝ is Riemann integrable on [c,d]. Prove that f\circ g is Riemann integrable on [a,b] if either f or g is a step function.

The proof for g being a step function seems easy enough, but the other way seems much trickier. Thoughts?
 
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Have you tried doing a formal composition:

ciχAi °g

And then extending by linearity to

Ʃk=1nckχAkog ?
 

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