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Full Idea
In 1936 Church showed that Principia Mathematica is undecidable if it is ω-consistent, and a year later Rosser showed that Peano Arithmetic is undecidable, and any consistent extension of it.
Gist of Idea
Both Principia Mathematica and Peano Arithmetic are undecidable
Source
Feferman / Feferman (Alfred Tarski: life and logic [2004], Int IV)
Book Ref
Feferman,S/Feferman,A.B.: 'Alfred Tarski: life and logic' [CUP 2008], p.193