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      Envelopes, indicators and conservativeness  [Article]

      Cordón Franco, Andrés; Fernández Margarit, Alejandro; Lara Martín, Francisco Félix (Wiley, 2006)
      A well known theorem proved (independently) by J. Paris and H. Friedman states that BΣn +1 (the fragment of Arithmetic given by the collection scheme restricted to Σn +1‐formulas) is a Πn +2‐conservative extension of IΣn ...