We introduced Robinson’s arithmetic 
We showed that
We proved the fixed point lemma and deduced from it Tarski’s theorem on the undefinability of truth, thus formally solving the liar’s paradox.
A kind of library
Andrés E. Caicedo
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Mathoverflow- Convergence of $\sum(n^3\sin^2n)^{-1}$
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- On the division paradox
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A kind of library 